There are three main opinions in regard to the length of time needed to be considered a new meal. Most Rishonim rule that the standard length of time between two meals is six hours, and that is how long one must wait between meat and dairy. This is the opinion of Rashi and the Rambam who say that it would take six hours for the taste of meat or the meat between the teeth to go away. Rabbeinu Yerucham writes that one only needs to wait three hours. (One possible reason is that Rabbeinu Yerucham lived in Germany where they ate five meals a day, so he interpreted the Gemara to mean three hours, for that was the length of time he had between meals.) The last opinion about waiting between meat and milk is to wait one hour.
Most poskim agree that one should wait six hours, among them the Shulchan Aruch, Rama, Taz, Shach, Pri Chadash, and many others. There are some that have a custom to wait less, and it is proper to follow one’s custom. (עיין שו”ע יו”ד פט:א)