Any answer depends on how much driving you do/how far you have to drive.
For me, it's about once a week during the wintertime to keep my tank at least half full all times, but up to ten days during the summertime when I can let it go down to a fourth of tank before heading back to the gas station (although it's been said doing that is bad in any kind of weather). The most I've ever spent at the pump at any one time was $25 USD.
My very first car--a '87 Plymouth Reliant--was a granny car with insane gas mileage after a mechanic did something with it that he was very proud of but I never fully understood. That little car was getting 35 miles to the gallon, and that was back in the '90s.