Yesterday I was out all day, Power Cover and Surface Pro 2, Apps Outlook, Lync, PowerPoint, Word and Excel also did some browsing in IE.
Battery Power Only, meetings all day with sleep in between:
8AM - 6:30 PM
Power Cover - 0%
Surface Pro 2 - 38%
With that remaining I could have gone another hour to 90 minutes. I did remove the Power Cover in a couple of meeting to take notes which impacted battery life on the SP2.