The Unofficial Guide to Walt Disney World (UG) has a great time estimator, among other great features; the researchers for this book actually have real experience riding the buses, and timing them, so it's the "straight dope" as it were. We recommend the guide to anyone going to WDW and frequently have older copies out on loan to friends, keeping the most current in the house for handy reference. Those folks at the UG are big bad Disney geeks, just like the rest of us on this board. Len Testa of the UG will sometimes read the board and clarify things.
The one on-line site is woefully out of date. So for the real information I'd say get yourself the UG.