42mm SBSS - I took mine off this morning at 5:30am when I got home it had 5% left on the battery. I'm not complaining because I strapped it on at 10:30am the day before so that was a total of 19 hours, 1.5 hours with the Workout app running, several notifications, emails, texts, 2 phone calls, Bluetooth music playing from the Watch to my earbuds during a 45 minute workout, a couple of reminders I responded to, and probably some other things I can't remember right now. This was with all of the fearures running, no power reserve or shutting anything off to conserve the battery. The screen setting is also always on the brightest setting.
I wasn't sure it was going to make it through the night but it did. Those of you that make it DAYS on a single charge I don't know how you do it, maybe mine is defective but it gets me though the day so I'm OK with that.