We were told they start 1 1/2 hours before the park opens. I would check with the front desk when you check in to be sure. There was usually a CM at the bus stop each day. He was a character, too ... always asked my DW and I "Where are you KIDS going today?" It got us in the right mood to enjoy the park. He also gave us a fairly accurate estimate of how long a wait we'd have.
The buses leaving WL seemed to run very close to schedule -- just about every 20 mins.