Mixed 1L of Kodak Rapid Fixer (without the hardener, per what I've read here) and was storing it in a 1L bottle. So far, so good. Only problem is I left the cap off accidentally after developing my last roll. For a day. Thankfully, it was in a room where pets don't go. Otherwise I'd be feeling even worse.
Is this mix still usable? I've only put two rolls and maybe three prints in it.
The fixer won't likely be harmed by leaving the cap off that relatively short time.
However, it is unwise to use exactly the same working solution for both films and prints.
It is much better to mix up a litre for film, and another litre for prints, and to track separately their use.
Film and printing papers leave different byproducts in used fixer. And if you are printing on fibre based papers, they tend to leave other stuff as well.
I echo the comments about using a lower concentration for prints and keeping it in a separate bottle.
You can always perform a clearing test on a snippet of exposed (fogged) film eg. film leader. Take an inch or two of film from the beginning of a 35mm roll (assuming you use 35mm), place it in a container, pour over enough fixer to cover it...agitate by inverting or swilling every now and then and the film should clear - literally it goes from grey or brown to clear. If it does so in a minute or so (maybe 2 minutes for some films and fixers) then your fixer is good.
Rule of thumb....fix time is double clearing time.