If there is an air space (as there usually is) I don't know. But I do know this and it will be about the thousanth time I have stated this.
When I buy color concentrates I pour them into clear, high density plastic bottles (soda, juice, these are EVERYWHERE). They do not 'breathe'. The 'bad' part is that there can be NO airspace and you have to figure the volume you want (most bottles are 500ml or 1000ml but there are exceptions). I use glass marbles to take up the air space but the bottles that have sheer sides (some sodas) are a bit squeezable (not much). Tiny liquour bottles also work (metal caps) and are 50ml so that is a handy size.
If you do as I say you will never again have to ask the question 'how long'? I have diluted RA4 dev that is nine years old and is as good as new. The method works either with concentrates or working solution. But one WARNING: the Kodak dev/repl RT (roller transport) has a nasty part 'C' that will compromise the plastic integrity of the high density bottles I spoke about. Either store that part 'C' in glass or keep that part in original container. It does NOT have to be protected from air as it is simply something for high PH. - David Lyga