From the K&N site:
http://www.knfilters.com/faq.htm#19
20. How do I know when to clean the filter?
The general rule of thumb is that the filter needs to be cleaned when the dirt build up gets as thick as the wire mesh. The usual interval is 30,000 - 50,000 miles depending on driving conditions. We recommend that you check your filter about once a year in normal usage.
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