So I've been on a long trip this weekend and shortly after I left I realised I'd left the charger for my Gwatch at home! As soon as I realised (watch on about 80% on Friday morning) I went into the settings, turned off the 'always on' setting and put brightness to a minimum.

Overnight from Friday to Saturday I put the watch onto airplane mode, and over 6 hours it lost 2%. On Saturday morning it was still on 70% battery.

I did the same thing on Saturday to Sunday, and again, hardly any drain. Unfortunately, today is a really long day so I've left it on airplane mode the entire day just to keep it going.

At time of posting then it's been off the charger for exactly 36 hours, and, although I essentially had to turn it into a dumb watch, it's still got 12% left and it actually looks likely that it'll just about make it home when I finally get back (about another 5 hours from now)

Tldr: if you really really try then it doesn't even matter if you forget your charger!