Depends on what you mean by "fresh." I have tapped stout kegs that were 1 1/2 to 2 years old and they tasted great -- actually better than they did when they were kegged and originally carbonated. I have lagered lagers for months so they were clearer and cleaner than when kegged. I have had many a pale ale or IPA improve with weeks and even months on tap. At the same time, I have had IPA's lose their hoppiness (primarily aroma) in a matter of a few weeks on tap. I can't think of a single beer I ever thought went bad or stale over the time it took to drink it.