How long is to long, when your body is telling you to stop moving, because it is too painful to get up, or move down. When that happens, is time to call it quits, the best way possible. However, there is to much that the world has to offer, by that time people have grand kids, and are still eating at fine restaurants. The answer is, is never to long, bring on cybernetics, renewable body parts, any except the brain. It may sound out of a movie, but you would be surprise how much technology there is out there.