Our DC mixes hold strong for over a year in most cases.
WB inks tend to have a ph shift and that's what causes their life to end early.
Keeping additives out until press time helps a bit with DC ink and a lot with WB.
I looked into this recently and short of having a nice, climate controlled ink area free of uv and very clean handling tools and containers you can't get much further than that 6mo-1yr it seems.