Get a small piece of ordinary, undeveloped, unfixed film. You only need an inch or so. You can do this in ordinary room light.
Use a dropper or a toothpick, etc., to put a drop of fixer on the film and let it work for a minute or so until you get a small, clear circle on the film.
Drop the film into the fixer solution you want to test and stir it up so that the film is completely submerged.
Use a stopwatch to time how long it takes until you can't tell where the drop was. Stop the timer. This is your clearing time.
Fixing time is usually twice the clearing time.
First, do this test with fresh, unused fixer the first time you use it. Remember (write down) the amount of time it took to clear the film.
When your testing shows that it takes twice as long as it originally did, consider the fixer to be exhausted.
Store your used fixer in a separate closed container, away from everything else, in a safe place where nobody (kids or pets) can get into it.
When you have enough, dispose of it according to local regulations. I take mine to the Household Haz-Mat recycling day that's held in my town about two times a year.
I don't know if the Silver Magnet works on C-41 fixer but I don't see why you couldn't use SOME method of recovering silver.
If you can extract the silver, it would be worth doing.