5-6 years. After that, the console and graphics start feeling very old and boring for me by that time.

5 years first party support

5 years at the very least, 6 is a lot more preferable.

5 years is good for now. But as games become bigger and bigger, I think it needs to get to 8 minimum.

I fail to see your point. Are you talking development time and resources when you say "bigger"?

I think Aonuma said it best. If they were to make a new Zelda on NX instead of dual releasing Zelda, we wouldn't see that Zelda until the end of that console cycle much like Wii U. Bigger doesn't apply to all games. More like AAA games that increase in size and budget. I really don't see how you got lost, bigger things take more time to build if you want to do them right.

6 years than release a new console and support the old one 4 or 5 years more.

At least five years, maybe more. I just don't see a console being a wise investment if its lifetime is any less.

I know where you're coming from, but game devs have always adapted. A developer can chug out a game in 3 years, just like a developer could have a decade ago. Actually, many devs have become more efficient at releasing large games at a steady rate. So any change in the development time wouldn't be enough to warrant an increase in generation length like that.

5 years, or 6 at max. More than that and it becomes a struggle. Remember last gen when we were dying for new consoles to show up? Wii lasted 6 years, which is ok, PS3 7 years, which is a lot, and X360 8 years, which is too much. It showed... games ran at sub-HD resolution with horrible framerate most of the time and things also became too stale.