Recently I purchased some test strips to see how much dissolved silver was in the fix that I use for film developing and what was it's pH. The results shocked me. I had been using fix that had 2-3 times the recommended limits and certainly have not be fixing my film to archival standards for probably six months or more. So that got me to thinking how should I change my work flow so that I don't get into the same situation down the road. Besides testing the fix which is expensive and a pain, the simple answer is just to count the surface area of the film you are processing. The fix that I use is TF-4 Archival Fix from the Photographer's Formulary. You can also get it from Freestyle and B&H (store pick up only). What I like about TF-4 is that it is a non-hardening fixer which makes it excellent for prints that are to be toned or retouched. Moreover, it works great for stain developers like Pyrocat-HD. I use a Jobo to process my film so that pretty much means that I reuse a full liter of stock solution TF-4 (1+3) regardless of which format I'm processing. I develop 120, 4x5, 8x10, 11x14 and 14x17 formats so you can image keeping track of all this for the fix would be a nightmare. So I came up with a nifty laminated sheet where I just check off equivalent surface areas. I simple rule of thumb is that 1 roll of 35mm with 36 exposures, has roughly the same surface areas as 1 roll of 120mm Medium Format film, which has roughly the same surface areas as 1 sheet of 8x10 sheet film. 4x5 sheet film is a 1/4 of a sheet of 8x10, 11x14 roughly two times the surface area and 14x17 roughly three times. So if you keep track in equivalents you will exhaust your fix after 20 uses (or equivalents) of 1 litre of TF-4 stock solution (1+3). To be on the safe side I made it 19 for some margin of error (thinner or thicker negatives will affect the fix) although I imagine things would average out of that much film.
You can download a PDF of my sheet here.
Some other good rules to follow from a discussion on the Large Format Photography Forum: