Just test your fix before you start printing. Under safe lights, cut a strip of the paper you'll be using, and mark it with a pencil - several lines across the strip, so you have 4 lines and 5 divisions. Rinse the paper, and then set a timer and dip it in the fix. Try, say, 20 seconds to the first line, then 20 to the second - leave the last section for your tongs or fingers. So you'll have a strip that's been fixed for 80 secs, 60, 40 and 20, divided by lines. Rinse and turn the room lights on, make sure the strip gets exposed to the light.
Drop it in the developer. The first section to go pure white is your minimum fixing time. You should see the strip go from white to vaguely yellow to full black. (This also tests your developer - the unfixed sections should be deep black).
I tend to increase the minimum time by 50-100%, though for lith printing I dilute my fixer and stay close to the minimum time for the dilute fix (seems to hold more color).
If you're doing a lot of prints or doing big ones, do this every 5-10 prints or so - just takes a minute and no guessing, no test kits, etc.