Ilfotol and Photo Flo are recommended solutions to use...but not required to use...in film development. They're effectively really, really dilute soaps.

My bottle of Photo Flo isn't as old as tfb's - it's going on about 8 years - but I can tell you that it still looks the same, smells the same, has the same consistency, and doesn't have any particles in the liquid. So, I assume that it's just fine to continue using.

Its use, for me, is a single drop or two of the concentrate added to the developing tank filled with water and then swished around and then disposed of. If you decide to mix up a working solution, I'd treat it the same. Add what you need to cover your film, agitate, then dispose. When you've run out, make more.

With something like Photo Flo, you want it to be the last thing to touch the film. Using it and then returning it to the bottle could introduce contaminants to the batch (though unlikely). Because the stuff is so cheap and so little is ever used, it simply makes more sense to treat it as one shot.

Side note, I TA'ed my high school film class and, even given our absolute crap budget, still used Photo Flo as one shot. I'd simply mix up gallons of it in the morning and make it available to the class throughout the day, who were instructed in the above technique of cover, agitate, dispose.