How long is a piece of string?

Seriously, it very much depends on how and where you ride. If it's dry and dusty, you need to do it more often. From a personal standpoint (and I'd say my riding is pretty average for the UK) I do the air box filter (Stage 1) annually, which tends to be 5000-6000 miles. The stage 3 side cover I do at half that interval (twice a year/3000 miles) as it does tend to pick up more debris than the internal filter.

You can usually get a good idea of when to do it by looking at the dirty side and seeing whether the wire mesh is visible and/or whether the filter still has some visible colour (from the oil). If you can't see the mesh and the filter media is grey/black then it'll need a service.

Here's what DNA say on the matter:

"The servicing intervals depend on the environment that the vehicle is used. Once a year or every 15.000 to 30.000 km is suggested, to keep your DNA filter in top condition. An easy way to check if the filter needs cleaning, is to see if the wire mesh that that is on the dirty side, is visible.
If you can see the mesh, you can continue using the filter. If the mesh is totally covered in debris, you must clean the filter."

Although I wouldn't want to leave mine for 9,000 to 18,000 miles though!

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