My experience with Kodak RA4 dev: unopened bottles of concentrate go bad in around 2 years, mixed developer in plastic pop bottles squeezed to get out all the air is still good after 18 months.
When our baby boy was born I didn't get into the darkroom for a good 18 months. When I did my unopened dev concentrate was bad, however stuff I'd mixed up 18 months prior was good.

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