The ADO website (http://www.ado.com.mx
, in Spanish only) shows an overnight departure at 10:15 p.m. from the Cancún City bus terminal, arriving in Belize City about 8 hours later.
From Belize City, it departs the "Novelo's" terminal on West Collet Canal (ugh! -- wish they would find a different location) in Belize City at 8 p.m. and arrives at Cancún international airport around 8 hours later.
NOTE: Oddly, although the return bus goes to the Cancún airport, the bus FROM Cancún is only from the downtown bus station. The ADO site has both the airport and just Cancún, Q.R., which is the downtown terminal.
Note that Mexico observes daylight savings time, while Belize does not, so that makes a difference in arrival times.
Fare is shown as 474 pesos, about US$38.50 at current exchange rates.
The buses on this route are ADO's executive-style late model 44-passenger buses with reserved, reclining seats, A/C, toilet, videos, etc. Not sure if these buses have the in-seat chargers for laptops and cell phones like some ADO buses have. I don't think these are the 25- or so passenger buses that have 3-across business-class style lay-flat seats that ADO uses on some long-haul routes.
I suspect this info could change as the wrinkles in the new service are ironed out.