How long can hepatitis a live in a person's body, before he realizes it?

Which hepatitis? If talking viruses, hep a is gone once illness resolves; hep b can be asymptomatic and persist for a lifetime in both active and inactive forms; hep c can be present in your system the same way and even active disease may not present itself until profound liver damage has occurred (if at risk, get checked); hep d or delta agent is only present along with chronic hep b infection.