Great post. This is really really easy. Plus its good to pull it out and look for moisture build up (possible rust) near the fan inlet. I just did mine at 39K and it was pretty dirty. How often depends on where you live, how often you drive, how much dust is in your area, etc. But not a bad idea to check it with each oil change. It only takes a few minutes to get at it. And if you want to be anal, not a bad idea to hit it with a shopvac after each oil change.
This is a good visual DIY video from Bentley: