It's always been advised that 6 months from time of opening is the best use period. I've personally used inks up to 12 months without issue, but it depends on environment and other factors, of course. I wouldn't rely on inks that have been opened for longer than 12 months, even though I know of people who have used them without issue.
This is only from the opening/first use. When stored, unopened, they have long expiry dates and in practice I've used carts that were 4 or more years past their due date when first opened without issue.