Meat after Dairy
This masechet states that one must wash their mouth and rinse their hands when going from dairy to meat.
Chullin 104
According to this sefer, there is no required waiting time between dairy and meat.
Shulchan Aruch
According to the Taz, this is the amount of time one must wait after eating hard cheese before eating meat.
six hours
According to the Shach, hard cheese is defined by this characteristic.
Having the appearance of six-month-old cheese
This rabbi of Tnuva, whose position is also that of the OU and Rav Aharon Kotler, holds that cheese does not substantially age in the refrigerator and must be six months aged.
Rav Zev Weitman