How long can I wear my last Invisalign aligner without teeth starting to shift back to their original position?

You can wear your last invisalign tray as long as you want.  Some patients use it as a retainer.  It's safe to do so.  It takes about 2 weeks to get a response from invisalign.  Follow up with your dentist to see if you can get a few refinement trays.

You can wear your last aligner for quite awhile, but it is not the best situation in the event you break it, etc and it will discolor as well.  Your dentist should be able to make you a temporary retainer similar to but stronger than an aligner.  As far as your smile is concerned, if there are any teeth that are not completely aligned or are still slightly rotated,etc. it is considered normal to have a refinement and this can be anything from a few aligners to 20 or more.  Align does not charge for refinement for full cases and in our office we use refinement regularly at the end of complex cases to perfect your smile.