"Clear" just means it's not milky anymore, so you can see through it. For example, try to read some text looking through the film - keep a small distance (an inch, perhaps) between the text and film, this way the milkiness will prevent you from seeing through. Use enough light when doing the evaluation.
Milkiness is caused by silver halides, the light-sensitive element of the film, they are somewhat translucent but not transparent. Fixer's task is to remove them completely from the film. When the film is visually clear, the bulk of them are removed. This time is at least doubled to ensure that A) all of them are removed, B) also the byproducts are removed.
Dyes in some films are a completely different matter, and manufacturers suggest that a small tint is perfectly normal and does not cause any problems. Most of the tint should come off in normal fixing and washing. If it does not, it is more important to check if the fixing and washing both are sufficient -- the excess dye is more like an indicator of a problem, than a problem itself.
I think that you should primarily follow the instructions on the particular fixer you use for capacity - just keep book on how many films you have developed. Do it so that you have doubled the fixing time for the last film, or something like that. Or, you can do this: when your film takes twice as long to "clear visually" compared to first film (of same kind of type!) processed in same fixer, it's time to toss the fixer. For example, the first film may clear in two minutes, so fix for at least four minutes. Then, after you have processed around 10 - 15 films per liter (depends on fixer and films), it may clear in four minutes, so you fix for eight and then toss the fixer.