In my experience the older ie retro hub bearings seem to keep on going for a long time, all depends on how much use they get and in what condition as well though. ie. My retro bike gets out once a week on average, so the bearings will probably outlast me!
By comparison, my modern bike with Hope pro2 hubs munches through bearings in around 12 months, but then it gets far more use all year long, and the last set of bearings were decent quality too.
Either that or they don't make the sealed bearings like they used to !