Right now? 60 years. That way you can avoid most degenerative issues and end life with most of your faculties still there. Hopefully we can push the body degeneration starting point further and further as technology progresses, but right now, on average, it's untenable.
Well, this is a philosophical question. I updated the description. You said "Hopefully we can push the body degeneration" - the question is how much would you like to slow down the body degeneration process.
Well, it depends. That much's obvious, isn't it?
I'd say it depends on how long can we do important stuff and also have fun until we start inconveniencing others. As a famous thinker from my country used to say: "Old people, to the grave. Young people, to the making".
I don't particularly love transhumanism, but that's basically what you're asking me to ponder. What's its limit? The easy answer is that it has none. If death can be avoided, then everybody should- no, must be able to avoid it.
An optimal human life span in this situation would be as long as any individual person would like to live. I'd say that the theoretical limit would be set for 200ish years, and the pragmatical limit should be around 500~. Otherwise you'd end up with too many mouths and too little food, or too many bodies and too little space (not to mention the cemetery issue. Hey, there's a thought! As soon as we get the technology, start throwing dead bodies into space. Aside from the weird aesthetic we'd be creating for future space-travelers, there's no big issue with that, right?).
Finally, there's the small fact that once we have enough tech to control death, we'll probably also have enough tech for some other issues, like Artificial Intelligence, or cheap and fast Body Modification, or Space Travel. Whatever we have developed, our nº1 issue after figuring out how to make bodies last (i mean, aside from the obvious thought of how to make MINDS last so that you don't slowly go insane in your meat-cell), would be to get enough resources for the new, huge influx of population you're going to get in the next generation. Earth has been, after all, a non-stop succession of mass extintions. Nobody who was alive 150 years ago is alive now. And nobody who would have been alive 100 years before THAT was alive by then. The human race as a whole keeps perishing and perishing, but since the new humans look quite similar to the old ones, they can cohabit and somehow live in harmony, everything considered.
Bottom-line point is: How long would YOU like to live? It's safe to assume, since you're not an exceptional decision maker (who would behave and choose differently from the rest of humanity) that however long you desire to live, most of your kin will desire too.
Haha, I love the dead bodies to space idea. Every tombstone would be required to have an inscription. Something like "He never dreamed one day he'd fly. And look and him now."
I think there's also one thing worth considering. The shorter one lives, the more productive his life is. Doesn't matter if he's healthy both physically and mentally. If we lived 500 years, I believe most of us wouldn't have the motivation to do the "important" stuff, and we would devote our time mostly for fun. That would make a huge difference to the world.
I believe 200-300 would be the optimal time. Anyway, it's hard to predict what changes would that bring. They'd be significant, that's for sure.