If your connected to a docking station for monitor access then you don't need to worry about battery life. Other than that windows does not really take much power in laptop mode. Smartphones have surpassed low end laptops for a while in CPU / RAM. It's just not an x86 CPU.
If you mean the normal smartphone 2-3days standby + 6 hours use then that's a non issue with a standard desktop OS (Win, OS X, Linux). The problem in making a laptop OS usable with a smartphone form factor.
If you want 18+ hours watching YouTube then nobody does that in a cellphone form factor.
I'm skeptical, as my current iPhone 5S can't manage 2 full days without charging, usually in wifi range, and very little video. Mail, calendar, iMessage, light web, probably less than 20 minutes talk per day on average.
Granted, it's 2 years old (or so), but I have to believe that Apple developed iOS with an eye towards optimizing battery life rather than adapting the UI of OSX.
