So it's interesting to see the difference between the 42 and 38mm side by side.
My boyfriend and I bought S3 non LTE AW's at the same time, him 42mm, me 38mm (42 would just look ridiculous
on me and is just right for him).
I knew the battery life was different but I wasn't expecting such a difference. We have fairly similar usage throughout the day, my 38mm over the first full week last week consistently drained to about 50% by 11pm (charged until 7am-ish). Just not enough to know you'll get through another day, so I've charged my AW overnight every night except for last night. It was at 41% by 7am, and now (almost 3pm) is at 20%. So I've got 8 hours max to go which would actually get me to 11pm but I'm not so sure that will work. I will stick it out but I don't like the idea of it running out at the wrong moment.
The 42mm on the other hand has been good for two days, so taken off charger at 7am, used overnight and put on charger again around 11pm the next day with about 30%. Over the weekend he even managed three days with 10% left at the end of the third day. I've checked usage in the iPhone's watch apps and it's not like his usage is much lower than mine.
I was prepared for the idea that I would be charging my AW overnight (I'm not that interested in sleep tracking and I really don't like wearables while sleeping) and that's fine, I do the same with my iPhone and daily or every other day with my iPad, but I wasn't expecting this big a difference in battery life