Gzinsel, the general consensus is that you can use fixer until the silver content reaches some predetermined level. There are several sets of published recommendations for this; for commercial work a higher level is allowed, for maximum permanence a lower level.
Originally Posted by gzinsel
The problem is that it's difficult to know how much silver you've built up. This is why the general recommendation for counting the sheets. For a little better accuracy, you can purchase "silver-estimating papers." These are little test strips which you dip in the fixer, then compare the shade of the test paper to a reference; it gives you an approximate idea.
If you're not already using a 2-stage fixer, you should look into this. It lets you get a lot more use out of your fixer, with a bigger safety margin to boot.