The clip test is only good if you've got something to compare to. Right? Unless you know how fast fresh fixer works then you don't know if the current fixer is okay or so far gone it's almost dead. FWIW TF-2 never seems to change the time to fix. Well not enough for me to tell the difference. TF-4 is supposed to last even longer. So I'm not too keen on the clip test.
Is it still a good colour? No yellowing? Are you doing two bath fixing?
If TF-4 is really long lived like they claim it should be just fine.