I've worked on one other 'B' series engine from Mazda. It's recommended every 60 000 Miles or 100,000 Km. Actually, that is pretty much the standard for all 4 stroke petrol engines that utilize a standard timing belt.
A side note, I'm quite sure (but not 100%) that your specific engine is non-interference. Alot of the B series engines from mazda are. This means if the engine is running and your old belt snaps the internals of the engine won't suffer any damage.
Basically, there is enough clearance between the pistons and valves to prevent major damage (ie; they won't smash against eachother during it's cycle when a snapped belt occurs).
D/L the Gates brochure and check your engine. This is a good PDF to keep for your records! Gates belts are good