It says a length of time that's significant for the character...but what does that mean for someone who grew up a mostly normal human but then became immortal a few years before coming to the Tower? Technically, thousands of years could pass and he wouldn't age a day, but he is still wired to think of a hundred years as a very long time. Well, maybe less so because of the average human lifespan in his universe and the fact that he knows a guy who's been immortal for a hundred years...