if i had ultimate control of when it would end, i'd like to live for millenia. (assuming i could lock my age where it currently is) there are so many things to do in this world, and nowhere near enough time to do them! i'd also love to see what happens to the human race over tens of thousands of years - where we would go, what kind of technology we develop, how our political/international relationships develop/grow, etc...
so yeah, millenia, as long as i could be in control of when i was done...