Never less than 1/4
Daddy always told me, "Kid. You can't trust your gas light. Never let your gas get below a 1/4 of a tank. Try to always keep it above 1/2. You never know when you're going to need it."
After I graduated from college and bought my first car, I generally responded, "Dad. I own a *modern* vehicle. It has a gas light for a reason."
And I never had a problem. Once, my gas light went on in the middle of nowhere in Northern California. I drove 30 miles to arrive, sputtering, at a gas station. I used this as my metric. I'd say to myself, "Sure, the gas looks low, but after the gas light goes on, I've still got 30 miles."
Last week, I ran out of gas on the freeway. My gas light never went on.
Guess that's just another thing my dad was right about...