I used an online Disney transportation calculator (don't remember what, you could google it.)
Magic Kingdom was the longest. The calculator said 22 minutes, others have said it was not that long.
Epcot/MGM were really quick - less than 15? Maybe 10?
Animal Kingdom around the same, 15 min.
Remember to figure in the time it takes to walk from your room to the bus stop, and then from 0-10 minutes (approx.) to wait for the bus. They usually come along about that pace.
I LOVE WDW transportation, and find that if you give yourself plenty of time to get anywhere, you should have NO problem with the Pop buses.
Once your bus actually gets moving, MK is about 15 minutes, Epcot is about 10-12, MGM is about 8-10, and AK is about 10-12. This varies somewhat with traffic and signals, of course.
And that does not take into account the wait for the bus to arrive, and the time it takes for the bus to unload and reload before getting on the road.