If I wear my aligners about 18/20 hrs/day, can I "make up" for lost time by adding a few days on to the end of my 2 week period?

Based on what you've described, it definitely does not seem like you would be doing anything horrible by making up the missed hours with extra days, however; it is a great idea to run your plan by your dentist to make sure that he/she is in agreement.  Ultimately, you want to maximize your results for the money that you have invested in Invisalign, so be careful to make sure that your doctor approves.  Good luck to you and I hope this helps. You can "Follow" me on Real Self to ask more questions.Sarah Thompson, DMD

The prescribed wear of your invisalign trays is 20-22hrs per day. You are correct, in that this can be a challenge sometimes. Missing a little bit of time (hours), is likely not a huge issue, but you should not miss a day(s) of wearing your aligners. If they start not fitting then your prescribed tooth movement may not track properly, and may even worsen. However, if you are concerned about having missed a bit of time, and you are supposed to move onto the next tray, you can always wear the set of aligners for an extra day or couple of days. This would slow your overall treatment down, but is certainly better than needing a revision, because the aligners are no longer fitting properly. Hopefully this helps!