Hello Andrew,
As a rough guide your fixer mixed for a litre of 1:4 should be good for at least 10 36 exposure or 120 films. I would probably discard it after 10 to be on the safe side. Test the clearing time of a piece of film in fresh fixer and if it is still close to this time after several films you willl be ok. The surest way of fixing, and the most economical in the long run, is to use two fixing baths: half the time in the first (older) bath followed by the remainder time in the second bath. After @ 6-8 films discard the first bath and replace that with the second and mix a fresh litre as the new second bath, and so on. This is the best way of reusing fixer whilst ensuring complete fixing. There are lots of threads about this and timing and capacity varies between practitioners, but I thought I'd point out the gist of a useful technique without getting too technical.
Regards Mark Walker.