I'm certain there is no such schedule, as drivers are dispatched form central locations based on need. During peak times near park openings and closing buses come very frequently to Pop more than one every 20 minutes, but the line for a bus may be long enough that you don't get on - esp. at park closing time. It's worth avoiding really peak times - like rightafter the parade finishes, but leave 30 - 15 minutes before the park closes - or push it and stay past park closing in the gift shops on Main St. at MK.
During non peak times , the wait at most reosrts is more like 30 miutes, though personally we had very good luck at Pop - since it's such a big resort, I think they run more frequently there.
I've only stayed at Pop once - but based on stays at a number of other WDW resorts, one of the worst bus wait periods seems to be around dinner time, trying to get back to a park, esp for the EMH park. Make sure to allow at least an hour to get from your resort to say a dinner at Chef Mickey's - esp. on EMH night. I'd really try to allow 75 minutes for that one, as you are supposed to check in early.
It also matters a bit how far your room is from the bus stop. I guess this is obvious, but if your room is far from the bus and you have to take the elevator - you'll have to allow extra time for that. It can take quite a while to travel around the larger resorts - and Disney often makes the pathways as indirect as they can.
