A general rule-of-thumb would require you to test your fixer working solution immediately after mixing to find its clearing time, then when it takes twice as long as the original test to clear the film, you may discard it. In practice, to test your working solution, you can take an unexposed piece of film (such as the film leader) and place a drop of fixer on the emulsion side of the film. Leave it for approximately 20-30 seconds. Then place the film into the fixer, ensuring it is covered. Time how long it takes for the clear spot to disappear (when you can no longer tell the spot you placed the drop of fixer from the rest of the completely clear film). This would be the fixer's clearing time. You would double this time amount to find the amount of time it takes to fix your film (i.e., if it took 2 minutes 10 seconds for the film to become completely clear and the dot to disappear, you would fix film for 4 minutes 20 seconds). You would test your fixer every time before using the fixer. When, during your test, it takes twice as long to fix your scrap film leader to clear as it did when you first mixed the working solution, it may be discarded and you may make a new working solution. (if you wish, you may wait until it takes triple the time your original test took to clear, but this would result in extremely long fixing times).

While it may be possible to use the same fixer for both film and paper, I do not recommend using the same working batch for both film and paper. If you do plan to use the same fixer for both film and paper prints, I would create one working batch to be used with your film, and a separate working batch to be used with prints.