The time given by Pat applies to plain hypo fixers (Sodium thiosulphate). I wouldn't want to keep using a rapid fixer that usually takes two minutes to fix a film when it takes three to clear one!(posted 8836 days ago)There is a safer method than this: Use test strips. They tell you how much silver there is in your fix. If there is too much (the upper limit is 8 g/l for films in ammonium thiosulphate based fixers), discard the fixer. For FB prints the limits are much stricter.