With fixer it is worth checking the amount of silver in it. It will eventually exhaust and cease to clear the undeveloped and fix the rest. You can buy test kits to do this. Fixer makers will also give recommendations in their literature.
For what it is worth a 250ml bottle of 1+4 fixer will do about 5-6 135 films before the test suggests dumping it but it is probably safer to stop at 4 films.
Others might suggest dumping much sooner on the " fixer is cheap, why save it and take any risk" basis but I rely on the test with a safety margin which give me 4 135 films
Same for 120 except that the bottles are bigger