How long will l have to wear invisalign for 18 trays?

At least 36 weeks: Usually invisalign trays are exchanged every two weeks. If the teeth do not move properly or if the trays need to be refined, the time you have to wear the trays will increase. Your provider should give you a good idea of how long you must wear the trays to achieve the desired correction.

36 weeks: I agree .. 2 weeks per tray, is usually recommended.

36 weeks: Each invisalign aligner is worn for two weeks.

Likely 36 weeks: typically you change trays every 2 weeks, but as this can vary I would trust the time estimate that your orthodontist provided. A good orthodontist will continually evaluate your progress and make adjustments as needed. If you went to someone who just handed you 18 trays-- good luck-- your in trouble! See if you can hand them back and get a refund and then go to a real orthodontist!