In the 1:55 time frame namely about 3 hours in advance. (4 hours for charter or international)
And your pre-preplanning should be for you to be back at your resort an hour before that pickup time. You must be at the platform -- children, carry-ons, everything -- with you 15 minutes beofre the pickup time.
You should get a printout/reconfirmation of your Magical Express bus back to the airport at your room door the day before checkout. If this is missing or incorrect, get a correct one at the front desk before beginning the day's activities. Caution: A bus time much less than 3 hours before plane departure is a Disney goof, not bonus park time.