For folks with Lehman 120s be aware that these engines pull so little fuel that it's likely a vacuum gauge--certainly the Racor gauge mounted on the filter itself-- will never show any reading at all under normal operation. So if the day comes when it does show a reading of any sort, that would be the time to change the filter element.
However, if one changes their filter elements on a regular schedule-- every oil change, once a year, etc-- the chances are an element will have been changed long before it reached the point of providing enough resistance to put an indication on the vacuum gauge.