Here is what I did (and do off and on). First, I charge the battery upto 100%. I use my Surface exactly as I would on any normal day (which involves browsing (min. 3 tabs open all the time), Office (with at least 2 documents open - usually Word and PPT), Mail (with 3 accounts checking mails every 15 minutes), WIFI and BT on all the time, SkyDrive open. I also spend about 30 minutes going through Flipboard. I time myself - at least the start time. I check the battery icon at irregular intervals - but more often towards the end of the day. I stop doing all this when I reach 20% (as per battery icon) but you could run it down till you get the warning from the system. Oh...I forgot to mention, brightness is at about 40%. This will give you a fair idea as to how long your battery will last under your specific use condition. I don't put too much faith in the battery tests that one reads about in articles/ blogs etc. because it is an artificial measure and each one's usage is different from that of others. Of course, YMMV.