In real world usage, after the initial geek factor expires, the device will probably last longer. I've certainly had that experience with the other Android Wear watches and Google Glass.
I'd also expect the upgrade to Android L to improve battery life.
I would expect a smartwatch's display to always be "on", displaying a watch face.
Edit: Nope, I am wrong - the screen does indeed stay on all the time, sorry.
"Like both already-released Android Wear watches, Moto 360 will feature an always-on screen, which is a power-saving dimmed display that still tells the time like a traditional watch." - http://www.techradar.com/news/portable-devices/moto-360-rele...
"One of the cool things about Android Wear smartwatches so far has been the always-on dimmed mode, so you can always flip up and see what time it is. The Moto 360, however, doesn't roll that way. When the screen's not on, the screen's not on. The one exception is when it's on the dock, charging." - http://www.androidcentral.com/moto-360-hands-redux