from K&N website:
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.
