I have a litre of d-76 left that is about 9 months old, stored in a plastic water bottle (club soda) with all the air squeezed out. That is actually pretty good for me, as I typically have three litres left in that amount of time. I tend to develop in spurts.
A cloudy precipitate is forming. Last month I'd used some and it was fine, but I was only testing a Bronica GS-1 back I found on ebay. I know it should be okay, but it does make me a bit worried that untouched developer is forming white filaments. Perhaps I should filter and not worry, but I'll use up the rest for testing too (I've purchased three more backs).
Soon I will be storing everything in glass, starting with the Diafine. It will make displacing air a bit more complex for developers that need it, but I've decided I like glass better.