We have used ART twice for trips to DL and used it for MM as well.
The first time (way back in 2009) we caught a shuttle at 6:15 and were in line by 6:30. The second time (2013) we caught a shuttle at 6:45 (it was the earlier on the schedule at our stop) and were in line right when they opened the gates at 7 am. Both times we stayed at the Homewood Suites.
I do remember thinking in 2013 that they had changed the schedule and didn't have an earlier shuttle like I wanted but it worked out fine. If you have specific questions you can call the hotel and they will know when the shuttle comes (if it has a stop at your specific hotel).