We stayed in the Hotel Pamporovo last week which is the last pickup on the ski bus route.
Most mornings we were up and out by 9:10am to find an empty bus waiting for us.
BE WARNED: the morning that we went out late (9:25am) we watched four full buses go straight past us. When we did finally get one, by the time we had collected our gear the queue for the 3-man lift was huge.
Moral of the story : don't be lazy - go and catch those early buses.