I'm curious too. Logic would have told me the alcohol would preserve the scent. If anything I'd assume further that a minute amount of the solvent would flash off making the scent slightly more pure. Sales Girls tell me one year. A nice bottle of Cologne is not worth 200 to me if it has a shelf life as it takes several years for me to deplete half of the bottle. I buy more than I wear too. My old colognes still smell great, but I don't know for certain that they don't smell different either.
My little brother swears that several of his bottles did go bad. They have no smell to them other than alcohol, but he would wear stuff from Hollister or other teenager smelling fragrances.