Quote from my site:

Restriction can be checked at idle speeds and at 2-3,000 RPM. Typically we will see approximately 2-3"hg at idle with very little variance as RPM increases. As a filter loads up, you will see restriction increase. When you hit about 5-6" hg (2" above baseline) I consider the filter well used and due for replacement. The early Dmax single layer media filters would run a long time without much of an increase in restriction. The latest dual media units seem to be doing a much better job of collecting dirt, but start at a higher restriction that rises quickly as they load up. Having this gauge on hand allows you to quickly and easily verify the condition and can be very beneficial when diagnosing fuel system related drivability issues. Typical OE filter life is < 15,000 miles. We have seen these filters load as early as 2500 miles or less depending on fuel quality.