It's such a simple process, it wouldn't hurt to do this every 15K. I'm kind of ashamed to admit this, but I have driven my 240D 100K before without checking them. That engine went 380,000 before overhaul.
In spite of this, unless you are buisy and drive 50K or more a year like I do, it would be a good idea to check them often.
This is a good example of "do as I say, not as I do".