I'm running at 75% brightness, battery saver is not on. I've been on battery for 5.5 hours now watching videos and surfing the web. Took one break where my surface book went to sleep (set to sleep after 30 minutes) but I was only way for just over 30 minutes so it wasn't sleeping long.
I'm showing 28% (battery #1) and 19% (battery #2) with a calculated remaining time of 1 hour and 51 minutes.
I haven't had my battery fail on me yet but I've never needed it for much longer than 5 hours between being connected anyway. I usually have it on a docking station all day at work and then use it on battery when at home. I only end up plugging it in at home on weekends with the way I use it so I've been very happy.
Update: at 6 hours the battery saver kicked in. Saying I have 54 minutes left now so I burned 1 hour of calculated time in 30 minutes. My screen is still at 75%.