Just drink 'em. That's what they're for. While they may well keep for a long time I doubt that they'll actually improve significantly. As someone mentioned, they're already several months old by the time you gt them.
Years ago we bought a few bottles (unlabeled blue cap (dubbel) and yellow cap (trippel)) right from the monestary at Westvleteren. We had some immediately, and hauled maybe six bottles back to Indiana. A couple of those bottles were stashed for maybe 3 years. While a side by side wasn't performed (how could it be) I think the 3 year old bottles probably weren't as good, or at least not as good as I recalled.
Unless some of the beer you're stashing is unique and irreplaceable then there's no particular reason for you not to enjoy them whenever you wish.