I mix up a 5L kit and it lasts me for months in a sealed bottle with no air. Used, I have kept chemistry for 2 - 3 months and fresh up to 6 months, but the key is NO AIR EXPOSURE.

Actually, I use Nitrogen gas as the 5 L bottle is used, and I store odds and ends in 1L or 1/2L bottles filled to the top.

PE