I try to keep them under ten minutes. It is a bit hard to cover many subjects in 3-6 minutes. But, the research says.... you lose attention over that time.
None other than upload size limit.
I would recommend staying between 3-8 minutes and not going above 10-12 minutes, at that point student interest starts dropping off.
IF you have long videos/topics, you can do part 1, part 2, part 3 etc.