I think the standard approach would be to find a fixing time that is acceptable as your system becomes "exhausted," then always use that time. When you have fresher chemicals, the fixing time is on the long side, but who really cares?If I use it to exhaustion, as determined by testing, will I be able to count on using the same fixing times for film and and paper right up to the point of exhaustion or do I need to make any adjustments as I approach that point?
The trickier thing is deciding what "exhaustion" actually is. The easiest thing is to follow the procedure spec'd out by the manufacturer of your materials, assuming you can find such a thing. Otherwise, the responsibility falls on you.
That said, I pretty much agree with John (and Gerald). The only thing I'd do differently is to check the silver concentration in the "clean" tank. I'm not up to date on this, but "silver estimating papers" used to be commonly available. These were typically used for roughly 1 to 10 grams/liter silver. You should check other sources for actual recommendations, but my fuzzy memory says that under about 1.5 g/l for film, and under about 1/2 g/l for fiber paper is fine for high-grade processing. Whenever either of 1) first fix time gets too long, or 2) last fix gets too much silver, then you bring in some fresh fixer and shift the tanks.
Counting the volume of material processed is a little iffy for B&W, since it actually depends on how much silver was developed. If you do mostly high key photos, then the great majority of the PAPER's silver must be removed in the fixer, whereas mostly black photos will barely use up the paper fixer. (For color materials, it doesn't matter since ALL of the silver ends up in the fixer.)
If you're interested in more details, I talked about replenished fixer systems in this thread, starting with post #31. http://www.apug.org/forums/forum37/1...l-fixer-4.html (I don't think you'll get any practical use out of it, but you might say "Oh, now I see why they're doing that.")
ps: Gerald's latest post, "fixer is cheap," is right. In real-world commercial processing, the main reason for multi-stage fixing is to maximize silver recovery and to help meet environmental regulations.