In my opinion, throw the stuff out after every use. You are then guaranteed that you aren't having bad prints because of depleted and dead chemistry.
When I print small prints (8x10) I use
Dev 9oz LPD 1:6 --1/5 of a bottle $9.50 or $1.90 worth
Stop Ilfostop 1/16 of a bottle @ $5.50 or worth $.35cents
Fix Hypam 250 of 5000ml @ $17.55 or worth $.87cents
Therefore I use $3.12 per session.
I hope the math is right but you can see saving this stuff is not really a great idea or savings, when you consider that with the developer the stuff seems to deteriorate overnight and with fix you have to be careful that you don't deplete the effectiveness. To me it is a false economy.
My opinion, throw it out and start fresh everytime.