Sheena, a K&N recharge is good for up to 50,000 miles on 'box' filters, but their conical ones can be good for up to 100K
Inspect your's annually. If you can still see the metal screen all around, then it's not dirty enough yet for a recharge, even if the cotton pleating is dirty
If you ever find any part of the screen is covered in dirt, then it's time
If you put on a lot of miles, it won't hurt to recharge it before it's due, but overdoing it is a waste as they need some dirt buildup to work more efficiently ??? That's their claim. So don't over clean it
http://www.knfilters.com/faq.htm
I put very little miles on my daily driver (>5K/yr), but I re-oil my filter every 20K or so because 'it wasn't red anymore'
My mileage may jump to 10K with the new job, but look how many years I could go between re-charges