We mentioned last week the requirement to wait after eating meat before eating dairy.  It is important to note that when calculating the 6 hours one counts from the time that one has finished eating meat until the time that one starts eating dairy (Shach YD 89:3 & Dagol Mirvava).  If one found some meat in between his teeth, he would not to wait from the time he removed it, rather from the time that he last swallowed meat (Shach YD 89:3).  If one has waited 6 hours he does not need to assume that there is some meat stuck in between his teeth (Badei Hashulchan 89:11).

The Shulchan Aruch [YD 89:1] rules one is required to wait 6 hours before eating dairy.  Many Poskim understand this to mean 6 full hours (Darkei Teshuva 89:6, Harav S. Z. Auerbach, zt"l [quoted in Sefer Doleh U'mashkeh pg. 257], Shu"T Mishneh Halachos 5:97:3, Badei Hashulchan 89:8, Laws of Kashrus pg. 200, Harav Chaim Kanievsky, shlit"a, quoted in Kashrus in the Kitchen pg. 15). There are Poskim that hold that one does not need to wait the complete 6 hours.  The source for this custom is not clear (see Shu"T Yabia Omer 1:YD:4, Laws of Kashrus pg. 200).  Some suggest this custom came about from the fact that some Rishonim seemingly allude to this idea.  The Rambam [Hil' Ma'achelos Assuros 9:28] writes that one needs to wait k'mo shaish sha'os (like 6 hours).  Similarly, the Meiri [Chullin 105a] writes 6 hours or close to that. 

The opinion of Harav Aharon Kotler, zt"l was that one only needs to wait 5� hours (this is the minhag in Lakewood Yeshiva).  The Chasam Sofer held that as long as one waited into the 6th hour [i.e. 5 +] one may eat dairy (Oz V'hadar Shulchan Aruch pg. 89:3).  Harav Moshe Feinstein, zt"l, held that one should wait 6 full hours.  However, b'shas had'chak (in a pressing situation) one may be lenient and wait 5 hours and 50 minutes (Oz V'hadar Shulchan Aruch 89:3).  The general consensus of most Poskim that one needs to wait 6 hours (See Rama 89:1, Shach 89:8, Taz 89:2, Chochmas Adom 40:13, Aruch Hashulchan 89:7).

There is a well-known custom of German Jewry to wait 3 hours.  Although there is a mention of this opinion in the Rishonim, it is not clear where this time originated from.  Some explain that in these communities, during the short winter days, they would eat their meals 3 hours apart (Darkei Teshuva 89:6).  Others explain that the wealthy people of those communities would eat 3 hours apart (Shu"T Pas Sadecha YD 29).  A third explanation is, in those communities they used to eat 5 meals each day, and each meal was 3 hours apart (Harav Yisroel Belsky, shlit"a, quoted in Halachically Speaking vol. 5 Issue 5).  Harav Shimon Schwab, zt"l, suggests that the source of this custom is they originally kept 1 hour [as mentioned by the Rama], however since the Gemara [Pesachim 12a] teaches that people miscalculate time by 2 hours, they added an additional 2 hours.  Harav Schwab, zt"l, added that the Rabbanim of Germany always kept 6 full hours (Rabbi Biberfeld, shlit"a).

The Rama [89:1] writes the custom in his times was to wait 1 hour after meat.  This is the custom of Dutch Jews.  This opinion is based on the opinion of Tosfos that holds one is not required to wait any amount of time; rather one cannot be participating in a meat meal.  The custom seemed to evolve that people took upon themselves to wait 1 hour [perhaps for kabbalistic reasons alluded to in a Zohar].  It is important to note that according to the Rama, the strict letter of the law is that one does not have to wait at all, however the minhag became to wait [preferably 6 hours].  Therefore, in pressing situations [as long as one recited a brachah acharona], there is room to be lenient.  Practical examples may include older people, pregnant or nursing women, or people who are sick would only need to wait 1 hour and clean out their mouth (Chochmas Adom 40:13, Aruch Hashulchan 89:7).  According to the strict letter of the law, children are not required to wait between meat and milk.  However, from a chinuch (educational) standpoint it is proper to train them in these halachos.  There are different opinions when to start.  One should consult their halachik authority.

In a situation that one is not sure about when the 6 hours have passed, there is a machlokes whether one needs to be certain that 6 hours passed.  Practically, one may rely on the lenient opinion (Badei Hashulchan 89:9).  Some Poskim hold, that even if one has the family minhag to wait less than 6 hours, it is still befitting to hold 6 hours (Harav S.Z.Auerbach, zt"l, quoted in Sefer Kitzur Shulchan Aruch Hilchos Basar B'Chalav pg. 113 ftnt. 19).  Others hold that one should not change his custom (Rabbi Yisroel Belsky, shlit"a, quoted in Halachically Speaking vol. 5 Issue 5).