"All Day" where "all day" is defined as 18 hours. My days are longer, as in I am awake longer than that.
I imagine the real world life is going to depend how much you use it. Arguably, it's pretty pointless, so it might well do. But so far the only people I've seen with "smart" watches, of any description seem to have them notify them of just about everything. Which means it won't last long.
Whilst there are undoubtedly fringe reasons for all the stuff it can do, I'm not sure it's so "useful" as pitched. Plus I wanna know... if the girl who left her key behind couldn't get into the garage or whatever it was, why didn't *she* have an Apple Watch to unlock it herself if they're so essential everyone has to have one (which is a paraphrasing of the way Apple promoted it).