I agree with the others that sealed jugs should be good for a long time. I just used a can of Powermaster that was a fun-fly prize from 5 years ago and it ran fine.

If I was not positive that the "old" fuel is good, I wouldn't even consider mixing it with "new." If it is "bad," and you mix it with "good," you end up with two gallons of "bad" compared to having one of each.