There's really no way to answer this.....it could be one minute, it could be a half-hour.
It depends on the time of day, time of year, how full the resort is. It depends on whether you happened to get to the bus stop just as a bus to your destination is arriving, or whether you walked up to the stop just as the bus was pulling away.
Disney pretty much dispatches buses on an as-needed basis. They run continuously throughout the day from Pop to all 4 theme parks, both water parks, and DTD. Pop's service is pretty much the same as any other resort. On average, you shouldn't have to wait more than 15 minutes for a bus.