All day. 10-12 hour battery life easily.

I also opened the chassis and added Liquid Metal (Grizzly) TIM. The little i7 stays pegged at 4 GHz without throttling and presumably also improves battery life.

I can run all day using Arch + vim, running tests, etc. Roughly 10-12 hours on my X1 Carbon Gen 5

If you're willing to afford some depth to your laptop, a dual battery T400 series will get you 12+ hours of video.