I've seen people post this type of thing numerous times over the last 5+ years, so during our 2015 trip, I kept track of how long we waited for buses. We were there in late June -- so a busy time. We use the buses exclusively for all of our transportation. We use the buses at least 4 times each day, sometimes more. Our findings were that 90% of the time we waited less than 10 minutes for a bus. The longer waits were almost always at the end of a night at park closing (when we would expect to wait longer) as well as during or immediately after a heavy rain storm when people are leaving the parks in droves. I kept track of wait times during our 2016 trip as well and found similar results.
We found that DS/DTD consistently had 15+ minute waits. We've also had terrible luck at Animal Kingdom Lodge. We've never stayed there but go there for a meal or just to look around on nearly every trip. We never had less than a 20 minute wait there and in 2016 we waited over an hour for a bus to DHS.
I think that sometimes the wait for a bus feels much longer than it actually is. There have been times that we're standing there in the heat after a long day of walking around the parks and it feels like we've been waiting for 20 minutes. Then I check the time and realize it's been 7 minutes. Exhaustion skews our perception of things. On my first trip to Disney as an adult (2001), we stayed at ASMo. I had done some reading before our trip and knew to be at the bus stops at least an hour before park opening in order to avoid long bus waits in the morning. I also knew that if we weren't attending the night time shows at a park, we should make sure to exit the park at least 15 minutes before the end of those shows to avoid those crowds. We felt the bus service was excellent that trip and rarely waited more than 5-10 minutes for a bus (I wrote a trip report but never posted it, so I documented the generalized wait times in that report. I would never be able to accurately remember something from 16 years ago). When we got home, I read a trip report from someone who was at ASMo the exact same days we were and he complained that the bus service was awful the whole week -- that he waited 20-30 minutes on a regular basis and always had to stand because the buses were so crowded. I found it interesting that we were there at the exact same time but had very different experiences. Timing is everything.