I only disabled watch mode last night when I went to bed. I'm at about 29 hours so far, and the battery meter looks like I have about 30% left. So still great life...but not the reported 48 hours. This was also using sleep mode last night, which I'm guessing bites into life a bit.
FYI, when you put it in sleep mode it turns off the screen anyway, so you don't have to disable watch mode.