For fixer, I make up enough film fixer to fill my largest developing tank twice.
I then fix in 1 batch for as long as it takes to clear a snip of film in the rest. Once that film is clear, I put aside the first batch and fix for the same clearing time in the second batch. Eventually all the used fix goes back into the jug.
I track the use by numbers of rolls and discard the used fix when the capacity is at about 75% of what the manufacturer recommends, or the clearing time doubles (whatever occurs first).