I use Ilford Rapid Fix, which only lasts (or, is only <i>meant</i> to last) a week once it's been diluted. I've never had issues with film not fixing properly (through any fault of the fixer at least!), I do, however, slowly increase the amount of fixing time as the days go by. Eg, on the day I mix it up, a 2 minute fix does the job, but if I don't use it for a few days, I might fix for 3 or so minutes the next run, at about a week, I'll go for the whole 5 minutes, plus a little more just to be safe. Like Pat said, fixer's cheap, so I'm never too fussed about mixing up a new batch if I'm suspicious.
Ian, interesting comment about fixing for twice as long as it takes to clear the film, I'm sure that tidbit will come in handy in the future!