I've been getting 7 to 8 hours, was consistently above 8 but I'm having issues at the moment.
The Marvell driver is making my Surface use loads of battery in connected standby. Refreshed it, it was fixed, 2 days later and it's back so I'm going to cave and call Surface support tomorrow.
Trouble is they are never fking open when I get home. Apparently they close at 7pm in my time zone, but when I go to the website it always says they are closed - at 6pm.
I'll try to post back with what they say about this issue, because it's a pretty bad one.