If you don't sufficiently fix films and/or prints, they will degrade rapidly in storage.
Rapid Fixer (Ammonium Thiosulphate) has a capacity of approximately 20 rolls per litre of stock solution after mixing at 1+4 from concentrate. One "roll" is defined as 80 square inches of film and is equivalent to a roll of 135/36, a roll of 120, half a roll of 220, 4 sheets of 4x5" or 1 sheet of 8x10".
TMAX films deplete fixer more rapidly, so you get 50-70% of the normal capacity when fixing these films compared to other films.
Get an offcut of a film, place it in a glass of freshly-mixed fixer and time how long it takes for the film to clear; it should be a bit under a minute. This time is T.
As the fixer approaches its nominal capacity, repeat the test with an offcut of the same type of film. Once the clearing time has reached 2*T, the fixer has reached capacity and should be discarded or sent for silver reclamation.
At any particular point in the fixer's life, you should be fixing for at least twice as long as the measured clearing time. With fresh fixer, that means you fix for 2*T and with near-exhausted fixer, you fix for 4*T.
TMAX films also take longer to fix.
Unless you go crazy and leave the film in for 20+ minutes, there's not really any such thing as overfixing. You need to fix to completion, so be conservative and give it some extra time if the fixer is doubtful. Fresh fixer can start bleaching an image (measurably but probably not noticeably) after about 10 minutes, so use that as a time ceiling.
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