What's it in? Your truck? How long has it been in there?
If it's anything 2-cycle, keep gas for as short as possible! The additives in gas these days suck and they break down really quickly. In a 2-cycle it's crucial to have good gas, and Stihl sent a memo to dealers telling them to inform customers to keep gas/oil mix no longer than 30 days /forums/images/graemlins/eek.gif That's a little short in my opinion...3 months would be about max I'd keep gas for a chainsaw, trimmer, etc.
If it's your truck, it really depends on how long it's been in there. Old/bad fuel just gums everything up and it wouldn't be a bad idea to run some fuel injector cleaner (lots) through the system. If the gas has been in there for a year or so, I'd say you're fine, but much longer than that I would think about draining it.
Oh, and for those reading this and thinking, "what am I supposed to do with the gas I bought 3 years ago for my chainsaw?" Well, see if your town has a place that will dispose of it for you. Or, the small engine mechanics where I work dump their old 50:1 mix into their trucks /forums/images/graemlins/crazy.gif /forums/images/graemlins/rolleyes.gif