There are two aspects to fixer 'exhaustion'.
The first is actual exhaustion of the thiosulfate. A commercial rapid fixer concentrate will generally be 50-60% ammonium thiosulfate, so 500ml of a 1+3 working stock will contain around 70g of ammonium thiosulfate. Now, let's do a little (haphazard/quick & dirty, paper napkin) calculations on this. B&W film contains up to 5g/m2 of silver, and a single roll is about 0.05m2 and thus would be 0.25g per film. The stoichiometry of fixing works out as around 1:2.5, so fixing 1 mole of silver halide will use around 2.5 moles of thiosulfate. Ammonium thiosulfate is around 148g/mol and silver bromide 188g/mol, so fixing 0.25g of silver bromide, or 1.33mmol AgBr, will require around 3.3mmol or so ammonium thiosulfate, which works out as roughly 0.5g. Since we have 70g of the stuff in our 500ml working strength fixer, you'd expect the capacity to be up to 140 films or so. Note that this is a very rough estimate and that the actual number will be quite a bit off depending on actual silver content of the film, fixing efficiency (i.e. thiosulfate availability), which fixing reactions actually take preference, allowed time to let the reaction complete, etc.
The second part is the silver content of the used fixer solution. Reusing fixer means that silver salts accumulate in the used fixer, and bathing consecutive rolls of film means you're effectively permeating the emulsion on these films with increasingly high concentrations of a silver solution. In principle this is not a problem, provided that these silver salts remain well soluble and sufficient washing follows fixing to remove them. Since a gelatin emulsion on a non-permeable base tends to wash relatively well, this is not a massive issue with film and RC paper, but it's a significant concern with fiber paper. However, even with film, it seems wise to limit how much silver you put into the emulsion (in principle an unnecessary activity, and detrimental to archival stability) as a result of fixer reuse. If you take 4g of silver as a reasonable upper limit of reused fixer for film (for paper, I'd stick to lower figures), this means that around 0.25g per film works out into 16 rolls of film per liter of fixer per liter of used fixer, or 8 rolls for 500ml.
The main takeaway from this, IMO, is that it's possible to make a rough estimate of a fixer's useful capacity, but that this estimate depends greatly on a range of assumptions, which easily tilt the balance significantly in either direction. Simply put: it's a good deal of guesswork and it's impossible to give hard and fast numbers. Fixer as such as a vast capacity, but the limits are posed mostly by how much silver you allow to accumulate in it before discarding, fixing speed (which tends to go down as the fixer is reused) etc.
What I've not addressed above is the problem of keeping jars of used fixer around and what that may do to your film. Everyone who has forgotten about a bottle of used fixer for a month or two knows that the silver will tend to plate out on the walls of the bottle and/or precipitate as tiny black particles. When reusing fixer that is in the process of doing so, these silver particles and flakes can easily embed themselves inside the emulsion and they prove to be very hard (or even impossible) to wash out once they're lodged in the gelatin. The net result is a firmament of white specks that will end up in every print or scan. It's crucial to only re-use fixer as long as it has not started to precipitate out sold particles.
Personally, I'd place the limit at 6-8 rolls per 500ml of 1+3 working solution when reusing fixer, and not keep used fixer around for more than maybe a week or two, depending on conditions (ah, more assumptions...). Although in my own use, I actually tend to mix film fixer at around 1+10 instead and mostly use it one shot, in a rotary setup for 135 and 120 films. This eliminates virtually all of the issues mentioned, and given the relatively low cost of fixer, it's not an expensive insurance against problems, either.
As anything in photography, seemingly simple things can be made very complicated. I think the trick is to simplify them again, in the end, and in this case, I do this by using film fixer one shot. It frees up mental capacity and shelf space for more important things.
Edit: corrected a pretty massive error in the capacity calculations; thanks
@Raghu Kuvempunagar for catching that one!