i have a timer but dont usually use it (unless im seeing how long i can stay in the air), just run the plane until you think your out of fuel, you will get a "feel" for it after a gallon or so
i usually fly until i feel like landing, or about ten min. i rarly fly at full throttle so i usually have half a tank left. however when im in the mood to just fly im good at guessing when the tank is almost out.
i fly close to the field so in the event of a dead stick i just land the plane (only happened once because of an empty tank)
since i dont have flight lines i dont need to maximize my flight times. if you do, i would go the timer method, then when the timer goes off gain some altitude, fly till you dead stick and land the plane.