What's your firmware version? This, more than anything else, seems to have a huge impact on battery life. That, and the fact that Samsung
screwed up the battery readings.
I don't know about video, but I did a stopwatch experiment:
The other day I tried to deep-discharge my phone (to the point where it couldn't complete a boot), then charge it fully while off, and then I started a stopwatch (with a keep-the-screen-on function) to see how long it lasted. After about 12 hours it still said it had 2 hours to go, but that was at 2am so I stopped the experiment a bit early. I used Ultrachron and Battery Left for this.
Since then my battery life has increased, it seems.