According to the Talmudic sage Mar Ukva, his father waited an entire day between eating meat and eating dairy in order to avoid transgressing the prohibition of eating milk and meat together. Mar Ukva, on the other hand, found that it was sufficient simply to wait until the next meal. This insight, which is recorded in Chullin 105a, is the basis for the current 6 hour waiting time observed by most traditional Jews between eating meat and eating milk. (As with all meat rules, this same standard is applied to chicken.)
While the different sages debated the exact length of time between meat and milk, once a certain standard was accepted within a community, that standard became the law. The majority of religious leaders throughout history understood the length of waiting time between meals to be six hours, and therefore, the majority of traditional Jews today continue to wait six hours. (A close custom, based on certain language of the Rambam,* accepts a waiting time of “into the sixth, meaning just over five or five and a half hours.)
Leaders of the early German Jewish community, however, set a three hour standard. It has been suggested that this standard came into practice because of the short winter day in Germany. Because of the early nightfall, many people ate their meals earlier and thus there were only three hours between major meals. The three hour waiting period continues to be the practice of most Jews of Germanic lineage.
Uniquely, the Dutch community accepted a standard of waiting only one hour.
No matter one’s custom, the period of time that one must wait after eating meat and before eating dairy begins from the moment one stops eating meat and not the time that the meal concluded.
*Rabbi Moses ben Maimon/Maimonides (12th century, Spain/Egypt).