I don't know if that fixer has hardener on not, but if it has, don't use it. In any case, it's fairly simple to check how much it takes to fix your film; just cut the leader and do the standard test. Put a drop of fixer on the leader and leave it there for 5 minutes. Then immerse the leader in the fixer solution, agitate intermittently and count the time it takes for the spot to disappear. Multiply that time by 3x and that's your base fixing time. You can leave it there longer without ill effects, in fact you need to fix for a massive amount of time to have a problem. Films clear in 30'' in a rapid fixer, but I fix for 5'. I never had any problems whatsoever...