Why do some people wear Invisalign trays for 2 weeks while others for 1 week?

It varies with the patient. Also how long you're wearing the trays. Most common tray transition is between 7-10 days. If you wear 22 hours a day and only take out to eat or brush most orthodontists allow you to wear a week at a time. You can consult at your next visit and ask your doctor if you can transition sooner, especially if you notice your trays getting loose after 7 days. If they are still tight the longer you wear them the more comfortable the process is and less likely to avoid teeth not tracking. Hope this helps!