Buses are posted as running every 30 minutes, however, in real life, they are dispatched every 15-17 minutes. Be aware though, because Old Key West is so quiet and peaceful, 15 minutes can often feel like it's a bit longer...
The buses are rarely crowded. Even during peak times, I don't often have anyone standing on my bus. Travel times are fairly decent... from the exit gate, 5 mins to Epcot, 11 mins to Magic Kingdom, 10 mins to Studios, 5 mins to Downtown Disney, and 13 mins to Animal Kingdom. (All are approximate times that I have encountered)