Do the clip test Steve outlines above. Get a benchmark clearing time for your film in fresh fix. However, be sure to discard the fixer when the clearing time is double that in fresh fix (much, much shorter than 10 minutes!).
Originally Posted by 250swb
Before each batch, determine your fixing time for that batch by tripling the clearing time in the used fix (yes, I know most say doubling is enough, but with the presence of silver iodide in many modern emulsions, tripling is necessary to ensure proper fixation for these emulsions). I add 10% to account for fixer exhaustion during fixing. I also never fix for shorter than the manufacturer's recommended minimum time, even if the time indicated by the clearing test is shorter. And, FWIW, I like to use two-bath fixing for film too, which is likely overkill for many. At least do the other things
Oh yes - the agitation. Just agitate as you do in the developer; more frequent agitation won't speed up the process, as Gerald points out above.