the perma wash just use it once and discard
i use it for film and prints its all good ..
fixer is a different story
as matt said,
do a clip test ... the leader of you film
and test it
see how long it takes when it is fresh + unused
double that time, splitting between 2 baths
when it takes 2x the original time for bath #1
mix a new batch of #1, make number 2 #2 and put the new batch as #2
it's not the best idea to drain spent fixer your town probably has a household waste disposal day
and you can bring your spent fix there ... or you can harvest the silver from it first ...
see my signature for info for harvesting the silver ahead of your disposal ... 10% to apug !
good luck !