I just bought a Samsung Galaxy S2x yesterday i let it die before i charged it, this is the second time i have had to charge in in less than 24 hours, it has been off the charger for an hour and it is at 69 percent and i have barely used it I am not impressed for a first time Android User
Something may be eating up the battery in the background or it's a faulty unit. Before we go crazy returning the phone, try this for me:
Take out the battery and SIM card for about a minute. Put everything back in and start up the phone again.
After using some apps, hold down the home button and swipe away apps that were kept in memory. Then go Task Manager (same area) and go to the RAM tab. Select clear memory. Do this every so often to "refresh" the phone.
Also, if you can experiment with this for the next day:
Settings > Developer Options > "Limit background processes". Set this to 4 or 3. See if it improves battery life. I know it did for my S III.
Just to add to Jonathan's points, swipe from the top of the screen downward and make sure that Wi-Fi, GPS and Bluetooth tab/icons are not enabled and should be greyed out. If you swipe to the left in the same area, you'll see a Power saving tab/icon, tap to enable/disable it. Lower the brightness setting or have it set to Auto. In Settings you can adjust the Screen timeout to say one minute or less. As cool as live wallpapers look, I tend to go with just a plain static wallpaper.