I am not sure if the following works for an actual answer because I figure that it will take an actual chemist or at least a professional photographer that has many years of experience to tell. But since this is too long for a comment, I decided to post it as an attempt to an answer.
I tried googling for an answer to this question and the closest information that I found is in this site.
I then decided to give my old fixer a try on some freshly exposed film. I did not add more concentrate than the advised quantity, but I did set the fixing time to 2 or 3 minutes more than what the instructions advice for. I used the same film that was used several times while the fixer was still "fresh" and developed with the same developer. The film was also exposed in the same way as before. But I did not take the same pictures as before for comparison. These conditions may be too "sloppy" for scientific proof, but it is all that I could come up with (taking into account my limited spare time and material).
The old fixer appears to work with no discernible side effects. I did not check the film on a microscope, nor am I a professional so my eye may not be the most accurate. But as far as I can tell, the fixer still works.
Now the thing that I certainly cannot tell is if the film lifespan will be negatively affected. Only time can. Or again, a chemist/professional photographer with lots of experience in the matter.
Just tested my "old" developer again following the description by @BobT (I assumed the information was trustworthy). The clearing time was of only 1 minute so the whole fixing time would be about 3 minutes tops on a 1+4 dilution (Ilford recommends 5 minutes in total). No discernible side effects. Seems to be working just fine. At the time of testing, the developer was 11 months old and counting.