Actually it is unlikely a fish with Ich would live indefinitely since at some point or another the parasite would infect gill tissue and strangle the fish off from oxygen. Ich goes through a number of life stages and it can appear like your fish has cysts and has been cured only to re-appear worse then before.
Many people treat ich by feeding vitamins, better foods, garlic based additives etc and while this can work it certainly does not change the fact that once ich is in your water, has hosts it will be there until it is either killed or the tank runs farrow long enough to allow the parasite to finish its lifestyle. I just wrote an article on my website concerning all this, http://www.reefwise.org/ich.htm
- hope it can help!