i think usage patterns have changed since the previous iPhone's so it's hard to really determine battery life.
i wasn't really a heavy user when i had the 3G/3GS/4 and i'm sure a lot of people could say the same/similar thing as there weren't as many "power hungry" apps around. even facebook didn't eat up that much power since there were no videos and memes being posted every single second of every hour and there was no auto-play videos. it was all fb status' (basically twitter).
i hardly watched youtube videos on that tiny 3.5" display. all i did on those phones was text, light web browsing and facebook. if i had the same usage patterns from 2010 as i do today, my 6+ battery would probably last me a week without being charged and i'm not exaggerating.
i became a heavy user after owning the iPhone 5 for a while (instagram, twitter would kill my battery). now i use the 6+ and i love the battery life