Old Key West busses typically run every 15-17 minutes during normal travel times, and could potentially run 25 minutes apart during the slow times of the day... (however, this is rare.)
Old Key West is not a very busy resort, and most times of the day, the busses are not crowded. Even during peak times, I rarely if ever have any one stand on my bus. During midday, the bus usually runs empty... with maybe 2-5 people at most per trip, if I'm lucky.
Another thing to note... there are 5 stops within OKW... the Hospitality House is the last stop for all busses except Downtown Disney.