I've always been told that fuel is good up to two years. Personally, I would not run fuel past two years. Like many here are saying, just get a fresh quart or gallon of fuel. I would not run old fuel due to the stability of the mix decaying after time.
I have had some bad fuel in the past. In fact, I bought it still sealed in the container and ran it. The tune was poor and the temps were not constant. Performance sucked. I don't run old fuel but I do use it for removing permanent marker marks and paint in my classroom!