Fixer doesn't have an exhaustion point that's really easy to nail down. I depends more on the amount of residual silver you are willing to live with in your film/paper and the amount of safety margin you want in your fixing regime so that you don't use fixer that is too full of unwanted silver compounds to fix with.
Ideally, we could easily measure the amount of dissolved silver in a fixing bath accurately down to the small levels that are needed (e.g., 0.2g/l for optimum permanence for fiber-base prints). Unfortunately, this is not practical for the typical darkroom worker. This is why you need an adequate safety factor (read, you don't really want to work at the edge of exhaustion; it's just too risky if you want optimum permanence).
My solution to get the most of my fixer while still ensuring optimum fixing is to use two-bath fixation whenever possible, even for film.
For film, do a clip test on your first bath before each batch. I triple the time instead of doubling it; some modern films do take longer to fix due to the silver iodide in the emulsion, and a safety factor for film can't hurt, since it is on an impermeable base. This time I divide between the two baths. When the clearing time for the clip test is double that in fresh fix, bath 1 is sent to silver recovery and bath 2 takes its place. If you develop film regularly, then you can keep the system going for the full seven changes before the fixer is too old (see below).
For fiber-base prints, don't take any chances, especially if you want optimum permanence. First, try not to overfix; use a minimum time and a safety factor on capacity. I use throughput as a guide and discard bath 1 somewhat before the recommended capacity is reached just to be sure. You'll still have a lot of silver in bath 1 to recover if you do this. It's just much better to err on the side of caution here IMO. I use 36 8x10 per liter as a maximum throughput (slightly less than Ilford's recommendation). If you use another fixer, read the manufacturer's recommendation. Again, when your capacity for the first bath is reached, replace it with the second. This can also go through seven changes till you should discard both baths and start over. Or discard when the maximum age of the fixer working solution has been reached, whichever comes first. Ilford recommends the following for fixer lifespan: 6 months in full tightly capped bottles, 2 months in a tank or dish/tray with a floating lid, 1 month in a half full tightly capped bottle or 7 days in an open tray.
Test your system at the beginning and then regularly thereafter for residual silver using the Kodak ST-1 residual silver test to make sure your fixing to an acceptable standard (and do the HT-2 residual hypo test while you are at it to test if you are washing adequately as well). Both these tests are available from the Formulary (with different proprietary names, but the same tests).
The above will give you peace of mind, get the most out of your fixer and ensure you aren't sending unused or partially used fix to silver recovery.
Last edited by Doremus Scudder; 10-04-2013 at 11:15 AM. Click to view previous post history.