According to 1999 edition of "The Unofficial Guide to Walt Disney World."
resort name: time-to-MK, time-to-Epcot, time-to-MGM (in minutes)
CBR: 13-40, 12-45, 06-33
CSR: 11-31, 08-28, 07-27
PO : 11-36, 09-33, 09-34
1st time assumes that a bus is at your bus stop ready to go as you get on the bus. 2nd time assumes that the bus just left and you have to wait for the next bus.
IE: CBR to MK --> 13 to 40 minutes