It will be somewhere between 2 and 10 minutes (it varies by film).
To perform the test:
1) take a scrap of undeveloped film (35mm leader is fine);
2) put a small drop of working strength fixer on it and let it sit there until the film looks distinctly clearer where the drop is;
3) immerse the scrap of film in a small portion of working strength fixer and start your timer. Agitate it for 5 seconds every 30 seconds. Ensure that it remains submerged;
4) when the film appears clear and you can no longer tell where the drop was, note the time elapsed. That time is the "clearing time";
5) your fixing time is between two and three times the clearing time - twice is what most people use.
You should keep track of the clearing time for fixer you re-use. It will get longer as you fix more film. When it has effectively doubled, it is time to discard your fixer.
If you have the opportunity, you should investigate "two-bath fixing". Just do a search here on APUG.
PS: Kodak gives capacity information on this useful link: http://wwwcaen.kodak.com/global/en/p...3cf/e103cf.pdf