How often do you clean your K&N?

Dirty = Better filtering ability.

A dirty K&N has better filtering ability, but it's airflow will be reduced. K&N also recommends replacing the air filter after 25 washings.This is what K&N says on there web site:Service Interval

A K&N Filtercharger is a high-performance air filter, both in terms of air flow and filtration. However, the service interval can vary widely depending on the severity of the driving conditions. The service interval can be from 100 miles in a desert-racing environment to 50,000 miles for normal street use. The proper way to determine when an air filter needs service is with an air restriction gauge. Such a device is commonly used on heavy duty trucks and construction equipment. A restriction gauge, measures the pressure differential inside and outside the filter and gives the information in different forms of measurement. As the filter collects more and more dirt, the restriction value increases. At a predetermined point or rate of restriction, the filter is serviced. The maximum allowable restriction for a K&N Filtercharger is 15" of vacuum (water). If the restriction is allowed to go higher, the filter media might become so restricted that the element could distort allowing dirty air to bypass the filter and enter the engine. Conversely, cleaning a filter too often will shorten its serviceable life expectancy. Installing a restriction gauge will optimize service intervals and take the guess work out of your maintenance schedule.

Service Life

A K&N Filtercharger can be washed up to 25 times before it is considered no longer serviceable. Even though the filter may not show outward signs of deterioration, it should be replaced after 25 washings. Excessive washings will deplete the tiny cotton hairs that crisscross the openings. The resulting damage will allow tiny dust particles to pass through.