Stephane: You should use separate batches of fixer for film and paper, as they use up the fixer at different rates. What information I have been able to gather about how many films and prints can be fixed with rapid fixer is available on my site at http://unblinkingeye.com/Articles/Archival/archival.html.(posted 8317 days ago)Ilford does not give a specific quantity of film you can fix with their rapid fixes, but Kodak says that one quart (or one liter) of film-strength rapid fix will process 32 8x10 inch negatives and, if you calculate square inches, a roll of 35mm or a roll of 120 are about equal to an 8x10 inch sheet film. My practice is to be a bit conservative--I mix up 2 liters at film strength and process 40 rolls in it, then replace it. I figure Ilford fixer probably works about the same way Kodak's does, so I use it the same way.
Ilford says that a liter of Multigrade fixer at working strength will fix 50 8x10 inch prints. I couldn't find that information for their Hypam Universal Fixer, but I assume it is essentially the same. My practice is to always use two fixes of 2 liters each, at film strength. I process 60 prints, then discard the first fix. The second fix is transferred to become the first fix, and I mix a fresh batch for the second fix. I don't use the solution to its full capacity, but I am assured of proper fixing under almost any circumstance.