[FormulaMMM]

There isn't a fact based answer to your question as far as I know, i.e. no data suggesting that 8 months is safe, whereas 12 months is not, etc.

I'm a relatively frequent oil change guy. The 4-7k mls between changes has "worked" for me thus far over many years of M ownership.

If I was purchasing your car, one change/year and every 5k miles would be totally safe from my perspective. Actually my greater concern would be with so few miles annually, is the car being used for short trips where the engine isn't even up to temp. (Rhetorical, you don't have to answer that. Just input from a purchase perspective since resale seems to be a priority here.)