The Shulchan Aruch rules that after eating meat (or poultry), one must wait a period of six hours before partaking of dairy products.
There is some discussion among the Halachic authorities as to whether one must indeed wait a complete six hours between meat and dairy foods. The Rambam (Rabbi Moshe Ben Maimon, Spain-Egypt, 1135-1204), in formulating this Halacha (in Hilchot Ma'achalot Asurot, end of chapter 9), writes that one must wait a period of "like six hours" ("Kemo Shesh Sha'ot"). This might suggest that one may partake of dairy foods already during the sixth hour since his consumption of meat, before a full six hours have passed. On the basis of this inference, some Yeshivot have adopted the practice of serving a dairy supper five-and-a-half hours after a meat lunch. Yeshivot that find it necessary to rely on this leniency for schedule purposes may do so, but otherwise, one must wait six full hours in between the partaking of meat and dairy foods. (See Halichot Olam, Helek 7, page 29.)
From which point do we count the six hours?
The Kaf Ha'chayim writes that the six hours begin from the final moment when one puts meat in his mouth. Even if one continues his meal and eats other foods, he may begin counting the six hours from the moment he ate his last piece of meat.
If one is unsure whether or not the six hours have passed, may he partake of dairy foods?
A fundamental principle in Halacha states that "Safek De'Rabbanan Le'hakel," which means that when one encounters a situation of doubt concerning a law enacted by the Rabbis (as opposed to Torah law), he may act leniently. Therefore, since the requirement to wait six hours in between the consumption of meat and milk originates from the Sages, rather than from Torah law, one may be lenient in situations of uncertainty.
Must a child wait six hours in between the consumption of milk and dairy products?
Due to the general weakness of children, they may be fed dairy meals even within six hours after they have partaken of meat. Preferably, one must ensure that an hour has passed since they had eaten meat. This applies to children up to 9 years old. (Hacham Ovadia extends this leniency to sick people as well. (Halichot Olam, Helek 7, page 42)
Once a child reaches the age of twelve, he must wait six hours between meat and dairy foods, just like an adult, as part of his training for Mitzva observance.
Some Rabbis held that when Halacha speaks of waiting six "hours," it refers to a Halachic hour, or one-twelfth of the period of sunlight on a given day. According to this view, during the wintertime, when days are much shorter, one would not have to wait six full hours, but rather six shorter, Halachic hours, and, conversely, during the summer months, one would have to wait more than six hours. This position, however, is incorrect; an "hour" for purposes of this Halacha is defined as a period of sixty minutes, and thus the time of year will have no impact on the required waiting period between meat and dairy foods.
Summary: After eating meat or poultry, one may not eat dairy products for six hours that is, six periods of sixty minutes after the last moment when he ate meat. If one is unsure whether or not the six-hour period has elapsed, he may be lenient and partake of dairy products. Children may be fed dairy meals within six hours of eating meat, but preferably not before a period of one hour has elapsed since they partook of meat.
