Time based oil changes (as a backup to mileage) in gasoline vehicles are mostly to address oil contamination from the engine not getting hot enough to boil off fuel and combustion byproducts in the oil. It's a rough heuristic for drive type - more advanced oil life monitors know the actual drive cycles an engine is seeing and will adjust appropriately, but for a basic time-and/or-mileage schedule, it's more about picking some interval that gets the oil changed before the additive package is destroyed by combustion acids. If the car is driven only short distances and never gets a chance to fully warm up, six months is probably a reasonable interval. If it gets a monthly spin on the highway for an hour, two years is probably fine. Ideally, you'd be measuring total base and total acid levels and calculating change points based on that; this is common practice for large truck engines but for a gasoline car engine, an oil change can be cheaper than the lab tests. (I'm still in favor of having oil testing done on at least some changes, it's basically "routine bloodwork" for your car and can detect many problems early.)

Another upshot of this is that with rarely driven vehicles, you might as well use the cheapest oil which meets appropriate specs, because your oil changes are driven by additive depletion and oil contamination rather than breakdown of the oil itself.