Honda does not specify a particular hour interval to change the belt. When you're under the deck and inspecting/changing blades, inspect the belt for any fraying, missing chunks, or other damage and replace as needed. If you notice the self-propel performance is reduced, it could be a worn/damaged belt too.
The transmission does not have any internally-servicable parts. External bits like the pulley, belt guide, cover, etc. are replaceable, but really the only thing you can service on the transmission is the oil level, and if it's not leaking or indicating any problems, no reason to even check that. When filled at the factory, the oil should last for the life of the mower. You must remove the transmission / rear axle to check the level. It only holds 2.2 ounces (65cc) and uses standard engine oil as a fluid (SAE 10W-30)