PermaWash (hypoclear) is intended for only one use. Mix up what you need and toss after use.
Follow the surface area calculations for you fixer's life – this may take a bit of calculation if not given as the number of rolls (or sheets 4x5, 5x7, 8x10, etc.) per volume of working strength, as printed on the label. Pour used fix back into a dedicated container (like school), and keep a tally of films used (on) with it. Discard (recycle) before exhausted (i.e., be conservative).