I use epoxyed wood lids in various sizes for various sized trays (8X10, 11X14, 16X20, etc.) and this allows me to use the same developer the nex day. Then I toss it. If I don't have any more work to do the next day I toss it at the end of my first day's work session.
However, Dektol, in a dark lightproof bottle stored in a darkroom keeps for a long time before its color changes. I notice that, as color changes, in-tray developing times increase, and at that point I dump it.
The special darkroom bottles that can be compressed as fluid volume diminishes are the best answer. I used year old D76 kept in one such bottle and it worked just fine.
Ted