Mine is every 12 months regardless of the mileage. I have been averaging 6k km per year. Because I do a lot of short trips I prefer this interval. Performing any earlier is pouring money down the drain. It's a peace of mind.
2007 E92 6MT - Silverstone II with /// Performance Exhaust with ACM Test Pipes and BPM Sport Stage II Dev-Tune
2011 F25 X3 3.0d