The average (and Invisalign taught way) is 2 weeks for each Invisalign aligner tray. Some offices have rushed it and done one week each, but the cases relapsed very quickly. Three weeks is the most I would do, but even then that is a bit long.
I would agree with the maximum duration of 3 weeks per tray. Any longer than that is unheard of.. I am pretty sure, your dentist would have some positive thought behind doing this.