Both fixer and stop are pretty much useable until they are exhausted.(posted 8586 days ago)Kodak does publish shelf life numbers on their data sheets or in the B&W Darkroom Handbook.
What I do for fix is drop the cut off end of the film into some fix when I first mix it. I time how long to clear. I write this on the bottle and fix for twice as long as it took to clear. I do this everytime I use it. When the time to clear take twice as long as it did with fresh fixer, I mix fresh fixer. If the last batch of negatives is important, I will fix for a minute or two in fresh fixer.
For D-76 it lasts for 6 months in a full container and 2 months in a not full container. The collapsible containers may or may not help shelf life. The plastic is thin enough that oxygen can diffuse right through them. The best for longer term storage is glass. Get a selection of sizes and either store in a bunch of smaller bottles, or as you use it move it to smaller bottles.
I reuse fix and stop for film developing. For printing, I normally toss everything at the end of the session.