most vendors suggest a two year shelf life. I've tried to estimate my needs for less than two years. Even though the smaller quantities are proportionately more expensive, the savings over OEM inks are still great and I don't risk having ink go bad.
I have some Hobbicolors ink that is almost 2 years old, possibly older I am not sure. I don't seem to have any problems. Always keep the bottles tightly capped and store in the dark cool place.