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

I was supposed to be finished with my last aligner on April 8th. However I'm still wearing it now (1st May). I told my orthodontist that I was not 100 % happy with the results so she took new impressions and pictures and asked Invisalign if refinements would perfect my smile. She doesn't think it will. How long should it take to get a yes or no response? She has no idea. I'm afraid my aligner will break or my teeth might move back to their original position. How long can I wear my last aligner safely?