Here at PDQ we would recommend that you clean and re-oil your K&N air filter every 10,000-15,000 miles. In very dirty/dusty conditions you may need to refresh your air filter more often.
K&N have a range of specific cleaning products to ensure your filter is kept in the best possible condition...
Instructions on how to clean and re-oil your K&N can be found here: http://www.knfilters.com/cleaning.htm
Filter Oil - 1 gallon
Filter sealing grease - Small tube
Filer sealing grease - Large tube
Filter Cleaning Fluid - Pump Bottle
Filtercharger Service kit - with squeezy oil
Filter Cleaning Fluid - 1 gallon
Filtercharger Service kit - with aerosol oil
Small Filter Oil - Aerosol can
Large Filter Oil - Aerosol can
Small Filter Oil - Squeezy bottle