When do you replace a BMW 330i clutch?

When it begins to give problems, such as slipping. There is no fixed interval and the mileage for clutch replacement can have extreme variations, depending upon individual driver's clutch expertise, driving style, and driving profile, e.g., a BMW 330i that is driven mainly on 300 mile non-stop trips on Texas Interstates may last 200,000 miles, one driven exclusively in New York City may last 25,000 miles, and one raced on the track may last 1,000 miles or less.

So your clutch is a bit like your tires -- only replace it when it wears out or has a problem.