It may have changed...my understanding was every 30 minutes or when the bus fills, whichever comes first.
So if you arrive at 9:50, you will need to hang out and wait for MIL before you check in at the Welcome Center (they won't assign you to a bus till you are all there). As soon as MIL arrives, go to the Welcome Center, lower level terminal B. There is no need to go to baggage check provided you have all placed the
DCL tags on your luggage before departure from your home airport. The check in line will be short or non-existent and you will be given a character designation which will be stapled to your document booklet. You will either be directed to the bus right away or asked to wait in the general area until your character is called--then you walk to the bus and board. There are couches and chairs in the area around the welcome center to use while you wait and a restroom is not far.
After check in...you may immediately board and the bus depart. You may board and wait on the bus for a short time. (5-10 minutes) Or you may wait in the area around the welcome center for 15 minutes or so before they board the bus and depart. We have never been kept sitting on the bus for any significant time.