I researched this. When you are going to the parks, you are the first to be dropped off, when you leave the parks, you are the last to be dropped off, and that takes about 30 minutes. The good part is that when you leave the parks, one of the stops is near Gardenwalk and all the great restaurants, so you could plan a nice little dinner break.
I did not use the ART though. Found the walk to be quite doable, even with a very tired 6 year old who we gave piggy backs to after 12 hours in the park, even on day 7 he was able to make it back to the hotel, although he did fall asleep in the middle of dinner at Mimi's.
You can grab a taxi back for about $10 at the end of the day, but my hubby is too cheap for that. Other people send the strong ones to get the car and pick the others up.