so what goes "bad" in oil are the additives package, that provides protection against things like acid, suspension of particles in the oil and stuff like that.

the general rule of thumb is for regular oil, 5k miles or 6 months which ever comes first and for synthetics 10k miles or 1 year.

the reason being, if you don't drive a lot, you probably don't drive very far either.

running an engine for short drives, where the engine never hits operating temp, is very bad for the engine, as water vapor is never fully removed from the oil. This allows the acids to build up and then reduce the additive package effectiveness.

This however doesn't mean you MUST change it at these times.
It is just a guideline.

If for instance, you only drive it once a week, but you always drive it an hour each way or something, then you are probably fine for quite some time.

you could also have the oil tested.
however for most people, it would be the same testing it cost wise, as just changing the oil.

Typically the run 30 bucks to test, and an oil change for even synthetic only runs around 35 bucks, 40 with oil filter.