Yeah, well . . . I'm nosy. I also have pretty good relations with my tenants, which I realize is not always the case. And I don't live too far from my properties.
I do the yardwork, which gives me opportunity to check the exterior. I often meet the tenants coming home from work that way, too. (Coincidence? Hmmm . . . )
I also change the HVAC filters every month myself. (The tenant knows this before signing the lease.) I always give 24 hours notice, but usually they're fine with me coming in whether they're home or not. (Always verify!) And if they're home when I show up, I always ask if anything is broken, needs repair, etc. If I plan on doing anything that would inconvenience them (paint the porch, replace sidewalk) I let them know 'way in advance.
Twice a year, I change all the smoke/CO alarm batteries & double-check the thermostat batteries. I check all the plumbing connections & re-caulk bathrooms as needed. I also make sure all switches & receptacles are not loose or damaged.
So far, every tenant has actually been very appreciative of my attention to the property. One of the biggest problems around here is that we have a LOT of so-called "slumlords" who let their properties go to the point that if the city were to do an inspection, they would deny occupancy.
So the short answer is: once a month.