As long as they are unopened, you can keep them for as long as you like.

My parents have kept a cognac for over 50 years...and we recently opened it. One thing that is very noticeable in old bottles is that they tend to taste heavier and have a very strong alcoholic aroma. It's because over so many years the ingredients inside the bottles breaks down slowly...but that doesn't mean its bad, some people love the heavy aged taste of old spirits.