When you have a dealer maintenance contract they will stick to their own replacement intervals! However many who want to keep their cars long term prefer to change either at least once per year or down to 5-6k miles, in some cases this equals the annual mileage anyway.
There is nothing stopping anyone, even with a maintenance contract, of changing the oil or anything more frequently, just make sure you use the original BMW parts!!!!!
Always said I would go back to a 6 series