I do my car servicing on a screwy schedule. My car sits under the carport a lot because I don't go out much. (That's in a state of change right now, tho.) Anyway, usually with my car, it's every 5,000 or 6,000 miles or 3 months, whichever comes first.
Since my car sits there for so long, I won't take it in every 3 months - can't afford to do that and don't feel it's needed just because 3 months expired. So I watch the mileage pretty close, but also, If I have something coming up, like a vacation and I know I'm going to have to drive it a lot and it's used to sitting around doing nothing everyday, then I take it in for the special safety check.
I saw the engine for the first time in a couple years the other day. (I don't need to look in there as I wouldn't know how to fix anything anyway - at least not anymore - but I was surprised to see how clean that engine was - it looked like a brand new engine.)
When I lived in Washington State, I used to have my tires changed every November 1st and every April 1 (I think those were the dates) because of requirements by the state. But it was also a good way to keep my tires rotated, checked a lot, and it kept me from letting them deteriorate because I wasn't paying attention to them.
Have a great one, all!