i would mix fresh fixer and do a clip test.
find out how long it takes to clear a piece of film in room light
and do the same thing for your old fixer ... if it takes 2x the first test ( unused fix )
it is no good ... you can use your old fix for your first bath and use fresh fix to "clean up"

its not worth using spent old fixer to process film unless you know how spent it actually is ...

good luck processing all your film !
when i returned from visiting family overseas
i had about 30 or 40 assorted rolls of film ( 35mm, 120, color, chrome bw )
and a bunch of 4x5 sheet film too ... ( 30 sheets ? )
it goes quick once you dig in

just make sure you have room to hang / dry it !

- john