Generally once a year for a general inspection. You should have a standard check list for this to document findings. On it you should have things like:
House keeping
Window glass
Rodents
Plumbing/Water
Carpet
Smoke Detectors (Change the batteries too)
Drains
Toilets
You are trying to do a few things:
Document/correct on going damage
Find immediate issues that need addressed (water leaks, broken glass, infestations, house keeping issues etc) so you can issue lease violations or perform maintenance.
perform ongoing maintenance (furnace filters, smoke alarm batteries etc)
Document everything is fine hopefully!
The last one is key. When you have a tenant get behind on rent and retaliate by calling hud or the health department to say they have no working toilet, smoker detectors and there is a bed bug infestation you will be glad you did that check 30 days ago that said all was well!
Not to mention certain issues won't be reported to you but can be important to know about, specifically in a multi tenant building. If you go into an apartment to do a check and find bed bugs for example, while that sucks, it is something you want to know about before you have to treat an entire building.
The one thing I want to know here is if you have forced air HVAC you should be changing filters more frequently than you do an entire inspection.