IF you get to Belize International on time, you should make your flight (especially if you ask for a seat close to the front of the plane and make sure your papers are in order).
If you arrive late in Belize City, you may miss your flight to Placencia since this flight usually leaves on time (planes can't fly after dark in Belize without a special permit - and there are no runway lights at most airports, so you wouldn't want to be landing in the dark anyway).
If you do miss your flight, you can stay at the Embassy at the International Airport (don't expect too much), or go into Belize City (try Colton House or Grant Residence) - you can fly out of Municipal the next day for less than it would cost you to fly out of International.
A taxi into Belize City will cost you $25 - $30 BZD (for the cab, NOT per person). Cab to Municipal next day should cost you $5 BZD.
If you end up going into Belize City and flying out of Municipal the next day, make sure you confirm your seats with Tropic at International before you leave.