Use it in every can of gas I bring home.  I keep between 5 and 15 gallons on hand almost all of the time.  I never drain gas, just park it and it's ready to go in the spring.  ATVs, saw, weedwhacker, mowers, whatever.
I didn't use it in ONE batch of gas, just one.  It was last summer and HOT AND DRY, so my grass didn't grow and I didn't need to trim much, so the weedwhacker sat.  I finally needed it, and it started but wouldn't rev...gummed carb.  That was the final straw.  Never another gallon of gas without Sta-bil.  I'm a believer.