I always assume it's going to take about an hour and it usually does, though it has taken as long as an hour and a half when I've come in late in the evening. The trip itself usually takes 30-35 minutes, so it depends on whether or not you have to pick up luggage (I usually don't), how long it takes you to get to the DME pick-up area, whether or not you have your voucher, how long the lines are at the DME pick-up area, and whether or not you're the first person on the bus. If you get there when the bus is already nearly full, you'll be on your way sooner than if you're the first one on. Those are the times when it's taken about 90 minutes.