<font face="Verdana, Arial, Helvetica, sans-serif" size="2">FWIW, Cancun to the Belize border looks to be about a 3-4 hour bus ride. </font>
I think it may be more than that. Looks like minibus service from Cancun to Chetumel takes 5 to 6 hours (and costs $20).
http://www.ambergriscaye.com/pages/town/article28.html
You'd definitely want to go the minibus route rather than the normal buses.