I'm two weeks away from picking up my M3, but I'm pretty picky about my current car ('06 Accord), even though some would laugh at so much attention to a "regular" car.
I like my interior very clean. After anyone rides in it, there are usually dirt crumbs on the mats. I can't make it 24 hours without vacuuming those! I can go a few days on the drivers side, mainly cause the driver's mat is harder to see when I'm in it
But, still like it clean.
No trash is in my car, ever. Any receipts, trash, etc., gets cleaned out at the first stop. I keep the leather cleaned and conditioned during every wash (only hand washing).
The car has 72K miles, and I think the interior looks outstanding. I've seen cars a few months old that look much worse.