Thinkpad's battery is 42 Wh with a wifi surfing run-time of almost 7 hours [0], although it being sold as a 11 hours run-time. It hasn't a dedicated graphics chip.
Xiaomi's battery is 40 Wh and it sports dedicated graphics. Macbook Air's battery is 58 Wh and it has integrated graphics, lasting around 10 hours [1].
Assuming a similar consumption, Xiaomi's would last less than 7 hours for surfing. But it has dedicated graphics (NVidia 940M) with a consumption (30W) that doubles integrated options (Intel HD in Lenovo and Apple laptops). So, probably it will last 5 or 6 hours of just wifi surfing, even less.
[0] http://www.notebookcheck.net/Lenovo-ThinkPad-13-Ultrabook-Re...
[1] http://www.notebookcheck.net/Apple-MacBook-Air-13-2015-Noteb...