If you have a gasoline engine I would fill the tank and assuming 70 to 90 gallons add about 4 cans of SEAFOAM stabilizer additive.
Fact is.... I have just over half a tank.. Did the math and figure I may not gas up till about May.. (Actually I will sooner, but I'm going to build up my discounts first) So i seafoamed the rig.. I still drive it every 1.5 weeks (Average) but I only drive about 20 miles.. Just enough (on the hills around here) to get it good and warmed up. So I Seafoamed.
Come June.. I'll make a serious drive.. but June is "Long time off".