I actually get very good battery life out of my Samsung Chromebook (arm based model), I have regularly gotten over 3h on netflix, plus lots of web browsing in on one charge. Can't tell you the specifics as I usually surf the web a lot than watch netflux, surf web, etc. And than I usually charge the thing well before it gets too low on the battery, I don't think I let the thing hit below 20% charge while doing one of these netflix marathons.
And yes well I may praise the laptop for this, it does have some drawbacks. For video playback online may be good, playing local files are a bit spotty and your limited to googles software. What I mean is your lacking support for a LOT of file types, and severely limited in options of what you can do while playing those files. So to put it short for online media consumption it is good, for offline media consumption if that was your intent there are far better options out there.
And this thing is far from having the max performance your money can buy, but some of the perks it does have are the battery life, being completely silent, and no fan. Last 2 I consider big perks if you want to watch movies on the bed or the laptop in your lap as you don't have to worry about covering up the vent which heat up the laptop.
But if the last 2 things I mentioned about it being silent and no fan are things you don't care about. And you have some intent on using local files to watch movies. Avoid chromebooks dispite how much I love them they have their flaws.