I believe you meant "How long should you fix for," right?

I don't have an answer, but though that maybe your question was missunderstood.

these might help:

look at the "residual silver test" and "residual hypo test"
These should help you determine how long to fix and wash for. I haven't tried them yet, but I might, someday....

If this is not what you meant, and you just need to know how to test for fixer exhaustion, Photog Formulary has a product for that as well, although I use the "Edwal" equivalent (tests for silver content of fixer)

Hope this helps