1) There are three places where the Torah states that one should not cook meat and milk together (Shmos 23:19, 34:26 and Devarim 14:21). Chazal tell us that one verse is prohibiting cooking meat and milk together. The second verse is prohibiting eating meat and milk that was cooked together. And the final verse prohibits any enjoyment from milk and meat that was cooked together (Shulchan Aruch Y.D. 87:1).
2) Chazal decreed that one may not eat cheese after eating meat, but may eat meat after eating cheese. The Gemara (Chullin 105a) mentions that Mar Ukvah said that he would not eat milk and meat in the same meal; however, he would eat a dairy meal following a meat meal. The Rambam and others understand that Mar Ukveh was dictating how much time one must wait between meat and milk. And that is the amount of time between the morning meal and the evening meal. This time period is six hours. [See Biur Hagra]
3) There are two reasons given in the Rishonim as to the need to wait 6 hours. Rashi (Chullin 105a) explains that when one swallows meat, the fat of the meat leaves a fatty residue in the throat and the mouth for that amount of time. The Rambam (Ma’achelos Assuros 9:28) explains that some meat might get stuck in between the teeth, and for 6 hours it has the hallachic significance of meat. However, after 6 hours the stuck meat is not halachically considered meat anymore and therefore even if one would then eat dairy one would not be eating meat and dairy at the same time.
4) We rule stringently and are concerned for both opinions and one must wait as long as either Rashi or the Rambam would require you to do so. For example if one swallows a piece of meat whole [without chewing it] he would be required to wait 6 hours. For although the opinion of the Rambam is inapplicable (because the meat was not chewed and thus none would be stuck in between the teeth), since the opinion of Rashi is still applicable (as the meat was swallowed some of the fatty residue coats the throat) one must wait. Similarly, if one merely chews on meat, even if he did not swallow it one would still be obligated to wait 6 hours because according to the opinion of the Rambam there is a concern that some meat got stuck in between his teeth. Even though Rashi would only require a waiting period if there exists a fatty residue coating the throat which occurs only when swallowing the meat.
5) The consensus of the majority of poskim is that one must wait a full six hours (Chochmas Adam 40:13, Pischai Teshuvah 87:4, and Aruch Hashulchan 87:7). This is indeed the most prevalent custom amongst Klal Yisroel.
6) There are those that wait only 5 hours (and a little) there custom is most probably based upon the words of the Meiri (Magen Avos page 46). The Meiri, when addressing the obligation to wait in between meat and milk writes that one must wait “five or six hours”. Rav Ahron Kotler zt”l was of the opinion that one should wait five hours and thirty one minutes (custom of Lakewood see Ohr Yitzchak Y.D. 4). He felt that as long as one has waited a majority of the sixth hour one need not wait any longer. One should follow his or her custom.
7) If one is unsure whether he has waited a full six hours (for ex. he is not 100% sure when he finished his meal), the poskim debate whether one is required to wait until he has definitely waited six hours, or perhaps he may act leniently and assume that the six hours have passed (safek d’rabbanan l’hakel). Harav Yosef Shalom Elyashiv zt”l and ybc”l Harav Yitzchak Yosef shlit”a rule leniently. However, Harav Chaim Kanievsky shlit”a rules stringently. (See Darkei Teshuva 89:5, Dalsei Teshuva 2, Badei Hashulchan, Yalkut Yosef Kitzur Shulchan Aruch page 837 and Ohel Yaakov page 222) For normative halacha, a rav should be consulted.
8) Three Hours- Many German Jews have the custom of waiting merely three hours. This custom can be found in the works of Rabbeinu Yeruchem (Issur V’Heter 39). Some explain that in these communities during the short winter days, they would eat their meals 3 hours apart. (Darkei Teshuva 89:6). While others explain that in those communities they used to eat 5 meals each day, and each meal was 3 hours apart. (Rabbi Yisroel Belsky zt”l cited in Piskei Rav Belsky page 132).
9) Harav Belsky zt”l (ibid.) maintains that one who has this custom should not change his custom to waiting six hours. However, Harav Yitzchak Yosef shlit”a (Yalkut Yosef Kitzur Shulchan Aruch page 837) maintains that it is preferable to change one’s custom from three to six hours.
10) One hour- We stated earlier that Mar Ukveh required that one cannot eat dairy and meat in the same meal and one can only eat dairy in the meal following a meat meal. We cited that many Rishonim understood that to mean one must wait the amount of time one normally waits in between meals (i.e. six hours). However, Tosafos has a different interpretation of the words of Mar Ukveh. He feels that one does not need to wait a period of time; rather the milk and meat need to be served in different meals. As long as one recited a brachah achrona and cleared the table, one would be permitted to eat dairy immediately. The Taz writes that the custom of many Dutch Jews to wait one hour is based upon the ruling of Tosafos. They feel that one need not wait at all; the requirement to wait one hour is merely a stringency they placed upon themselves.
11) Harav Belsky zt”l explains that once a woman is married, she follows her husband’s custom in waiting the allotted time between meat and dairy. No hataras nedarim is required after she gets married and changes her minhag. (Piskei Harav Belsky zt”l page 134)
12) Rav Moshe Feinstein zt”l (Igros Moshe Y.D. 2:26) writes that one who swallows a meat vitamin (ex. A liver pill) does not need to wait six hours. He explains that the decree (to wait six hours) was never extended to meats that were not meant to be chewed. Additionally, since there were no pills during the time of Chazal, they were never included in the gezeira.
13) The Haflah writes that one who found meat in between his teeth and swallowed it [whether within the 6 hours or even after 6 hours from eating] would be required to wait 6 hours from that point. His opinion is cited by Rav Feivel Cohen shlit”a in the Sefer Badei Hashulchan (89:13 Tziyunim 22). See however the Chayei Halevi (5:60 note 5) for a permissible view.
14) The Sefer Vyaas Avraham writes a truly novel hallacha. He feels that one who sleeps after eating meat is allowed to eat dairy immediately, even if six hours have not past. However, most authorities do not rule leniently. The Chasam Sofer originally agreed with this approach. The Chasam Sofer once placed milk near his bed in order that after his sleep he would be able to drink it, even though six hours had not past. However, while he was sleeping he knocked over the milk. The Chasam Sofer accepted that as a sign from Heaven that the hallacha does not concur with the Vyaas Avraham and one needs to wait six full hours even if he slept in between (See Tshuvos VHanhagos 1:43). Harav Beslky zt”l and Harav Wosner zt”l also ruled that sleep does not exempt a person from waiting. (Piskei Rav Belsky zt”l page 134 and Koveitz M’Beis Levi 9 page 23) The common custom is to rule stringently.
15) Harav Yisroel Belsky zt”l ruled that it is permitted for a person to swoosh fleishig clear soup in one’s mounth then spit it out and there is no waiting required if he wants to eat dairy. However, one must clean his mouth out prior to eating dairy (Piskei Rav Belsky ibid.). Like all cases of halacha, with a slight variation the halacha may change. Therefore, one should only act leniently in the case of Harav Belsky- with clear soup that one spit out. The halacha would be different if one were to chew a meat dish and spit it out.
16) One who eats a parave food that was cooked in a clean meat pot (even if the pot had been used that day to cook meat), does not need to wait 6 hours (Rama 89:3).
17) The custom is to start counting the six hours when one finished eating meat, not from when the meal ends. For example, if one finished eating meat at 1:30 p.m., and the meal ended at 2:3o p.m., one begins counting six hours from 1:30 p.m. (Badei Hashulchan 89:7)
18) If an onion (or any other sharp food) is cut with a meat knife or sauteed in a meat pan those onions have the halachic status of meat. If those onions get mixed with a dairy dish the dish would be prohibited. Rav Akiva Eiger zt”l (on S.A. 3) rules that although these onions have the status of meat, one need not wait six hours after eating them.
19) The Pri Megadim (O.C. 494:6) explains that even Rav Akiva Eiger zt”l would agree that if one ate meat (example- hamburger) and is in middle of waiting six hours, he may not eat an onion (or any other sharp food) that was cut or cooked with a dairy knife or in a dairy pot. Since an onion that was sliced with a dairy knife has the status of dairy in halacha, one may not eat it during the six hours.
20) One who ate meat and then vomited must still wait six hours from the time he finished eating the meat. (Darkei Teshuva 89:9 and Piskei Rav Beslky zt”l page 133)
21) If one forgot that he was fleishig and ate dairy, one may not continue eating dairy. Rather, one must stop as soon as he realizes his mistake. (See Yalkut Yosef page 431 and Ohel Yaakov page 221)