No, they don't have a set schedule. They usually leave about every 20 minutes, or so I was told by my last driver. I thought they usually waited until they had a full van but this last time, as I was purchasing my ticket, the lady said that a van was getting ready to leave and they held it for me. When I got out there, it was just one family of three in the van. I said something about it the driver and she told me they like to leave every 20 minutes or so, even if they don't always have a full van, as they dont' want people waiting forever.
Each time I've taken it, the schedule of stops just depends on where each passenger is staying. But they do hit DD hotels first and then WDW. When I stayed at the Dolphin, I was the first stop of the WDW resorts. When we were at the WL, we were the last. I dnt' remember other times.