Here is what I do:
1) I keep one bottle for my film fixer - it is large enough to run at least two separate batches of film at the same time;
2) I pour one batch of fixer into my film developing tank at the start of the fixing stage, and do a clip test with T-Max film with the rest at the same time;
3) When the clip has cleared, I dump the first fixer into a graduate, and pour the second batch into the tank. I then fix for the same time as the first run, but this time in the somewhat fresher fixer;
4) After use, all the fixer goes back into the bottle - although I will consider filtering it if necessary.
I continue to use that fixer until the earlier of:
a) I have fixed enough film to have reached 2/3 of the manufacturer's recommended capacity for that volume; or
b) the clearing time reaches twice what the starting clearing time was,
at which time I take it over to friends' place, where they are reclaiming silver from their fixer (they have more space, and use more fixer).
I put masking tape on the bottle, and mark it with the initial clearing time and a "/" for each roll fixed.
If I had more room, I would use two separate bottles, and the two bath fixing regime.