First, developer is best used one-shot. Just mix up what you need for each session/roll/batch.
Second, if I'm not mistaken, Ilfostop is a citric-acid-based stop. Since citric acid is organic, it has a tendency to grow bacterial slime and doesn't keep well. I use citric acid stop one-shot most of the time. Mix up the minimum amount you need for a session. If you are planning to process on two (or maybe three) days in a row, then you can save it and re-use it. Otherwise, toss it. If you're just doing a small amount of film, you can mix it half-strength to stretch it a bit. Still, stop bath is pretty cheap, so when in doubt, throw it out.
An acetic acid stop like Kodak Indicator Stop will last much longer. It can be stored and used until it just starts to lose its yellow indicator color (don't use it till it turns blue - it loses its activity long before that). I used to keep a liter or so for film developing for months.
Fixer: Although many keep and re-use fixer for fixing film, I don't think it's such a good idea. I try to accumulate enough film so that I have a large enough batch to use up all or most of the capacity of at least 500ml of fix so that I can dispose of it after the batch. This is certainly "best practice" (as is two-bath fixing, even for film).
That said, if you do decide to re-use fix for film, mix up the smallest amount that you can practically get away with so you don't end up keeping it for very long. Keep track of the capacity and do clip tests to check for exhaustion before each session (search here for clip test if you're not familiar with it).
Hope this helps,
Doremus