Well it depends on where u live and how much dust/dirt you have in your area when driving. K&N usually recommends every 100K miles, but I remember cleaning mine and re-oiling it at 30K.
I have an AEM Dry Flow filter now and it stays pretty clean, I just cleaned it after 12K miles, but it didn't even need it.
As a general rule of thumb it all depends on how much use it got with the elements.
