According to tests done by gsmarena, the Nokia N9 managed good 8 hours and 40 minutes of video playback before its battery went from full to 10%. Below are their complete rankings comparing it to other smarphones:
Nokia N9 does 8 hours and 40 minutes of video playback on a single charge [TEST]
As for the testing procedure: we charged each handset until it said full and looped a standard definition XviD video with the screen brightness set to 50% until the battery level fell to 10% (beyond that point most of them automatically turn off the video player and don’t allow restarting it).
Source Gsm Arena