You aren’t going to get a definitive answer from anyone. A cheap 6ft cable can fail to provide adequate power and so can 20ft cables. I have 20ft cables daisy chained to 6ft cables on several cams and have no issues.
The reason there is no way to lock in a definitive length is because it depends on the cable construction. The size and quality of the wire used within the cable is the determining factor as to how far it will carry the current without loss. With the tens of thousands of possibilities out there in length and quality, it is impossible to put a number on it.
@Newshound did some phenomenal testing of some USB cables that may give you an insight into the variables.