Some developers get the one shot name but don’t deserve it. Like hc110. I like to mix a tank of it in dilution c 1+19 and use it for a month then toss it. I only developer sheet film and I will never run through the capacity of it before the month is up.
Here’s another data point for HC110. I have used 550 ml of HC110 B (1+31) as almost my only developer. I used to develop up to 3 rolls of 35mm film and a few sheets in an evening and then discarded it. In the last year I have been keeping it for up to a month in a 500 ml bottle for up to a month with no detectable problems (maybe 3 or 4 developing sessions) towards the end the bottle has an inch or so of air.
This is also starting from syrup that is years expired and darkish brown. Haven’t been trying to break any records; it probably lasts a bit longer.
A test before using it is to dip a piece of film in full light half into developer and note the time it takes the light wet film to return to the same darkness as the dry half
Ilford multigrade 1+9 paper developer lasts less well. Half a bottle of unmixed developer dies in a few years. A tray of 1+9 from a year old half bottle will behave differently at the end of a long day and will be dead if left uncovered overnight. I have stored fresh 1+9 developer in a full bottle overnight for a few days but make fresh test strips each day. You can’t trust a calibrated Analyser Pro or a test strip you made yesterday with this level of imprecision. I should probably throw away my current half bottle.
There is a noticeable difference between uncovered trays, half full bottles and bottles filled to overflowing