Since there is no dipstick on the BMW, I have to remember to run the "ask the car to check" procedure in the iDrive. I try to do it 2x a month.
The Regal, with its oil loss problems, needed a check every 2 weeks, and usually some oil added.
The Park Avenue burned a little oil, but it had a sensor to detect low oil and tell me. Still I checked 1x a month.
I guess I did once a month on most of my earlier cars.