Well, that would be a little difficult to say really, as I have developed both 120 and 35mm in it, always one film at a time and usually 500ml-600ml ready mixed at 1:20 (where I could find times for it).
Roughly, after checking my fixer bottle, I think we're talking about 13-17 120 films and around 10 35mm ones. (mind you, with 600ml in my tank, I can develop two 35mm films in one go, but I haven't had more than one film at a time, so..).
I have changed/exhausted fixer twice during this time, so the markings on the bottle are a bit iffy to read. =)
(I will need to check back the actual numbers)