6 months, like most useby dates is conservative. If they said 6 months and ink regularly had a problem after 3 months, they'd be in strife.
I obviously can't offer you any guarantees, but I would be surprised if you had problems up to 9 months unless you have some very adverse environmental issues, and probably OK up to 12. Of course, you won't be throwing out full cartridges and you don't have to toss them unless you see a problem.
What printer are you currently using and how much ink are you pushing through at the moment?