I don't own a BMW--unfortuantely, but i "used" to wash my Mustang (which was black) at least once a week and detail/wax every other week sorta like epj3.
Considering I'm doing body work on my car right now and it's all in primer i don't wash it at all =)
but once it's going to be complete, chances are i'll wash it after everytime i drive it.
I know this sounds lame, but washing your car is also spending time with your car. After you take it out and enjoy it for however long you drive it you bring it home and clean her down so she'll look good next time you go out.
Similiarily, if you go to the Gym and work out, you don't come home, change and then go on a date--you take a shower first (i hope so). Cars are the same way, if you truly love what you drive and are passionate about it, you'll take care of your car as much as you take care of yourself. I know that sounds a bit extreme and i am making quite an overstatement, but thats just to prove my point. Your car, in essence, is a representation of your character. If you wash your car once a year, i bet your house/place of occupancy is dirty, you probably have 4 foot long hair, and you're pre-ordering dentures.
just my 2 cents.