Depends on the composition of the blend and storage conditions. Usually, as per my experience, time will take its toll in the loss of top notes after five to ten years (or more) of storage, but I insist, it depends - for example, among fragrances owned by my Grandmother, a small flask of Shalimar extract dating from the 1930's smells perfectly well, while what is left of a bottle of eau de cologne has a dark brown sediment, so if you want to try it you have to shake it, then it would turn milky brown... ugly, but still nice smelling. On the other hand, a bottle of an eau de lavande went bad after five years: the oil got separated from the alcohol and the smell was literally that of rotten flowers.
Store the bottles in a dry place away from sunlight. Take in mind evaporation and concentration of essential oils, so use the content of the opened bottle as fast as possible.