It has about 15 days.... In the first times I used the wireless to do the updates (in the first cycle it drained 30% in 15 min), but now I only use Reader or OneNote... The only software I have besides the ones that came is Office... So, I imagine that with web browsing or other programs the battery will last for much less than now... However, it is still in the "evaluation" period so, if this is a defect, I can go to the store and try to get a new one...
Have you calibrated your battery? You should run it down until the device shuts off and then charge it fully without using it. If you haven't done this yet in the 15 days you have owned the device, you might even have to do it twice. Honestly, I've written this so many times now that someone should sticky it.
Start with one run and then run a battery report. Run Command Prompt as admin and then change directory to your user area by typing 'cd c:\users\user'. So for me I would type 'cd c:\users\liam'. This changes where your report will be saved - as the default is system32 and you can't open things there without moving them.
Now type 'powercfg /batteryreport' - this will show your battery usage and after calibrating your battery, it will show your correct battery capacity.
If you are still having issues after calibrating, you should also run another command, after changing to your user area (only have to change directory once each time you run the command prompt) - type 'powercfg /energy' - this will check the efficiency of your system.
It's also possible that you could have some third party software installed that is killing your battery, so let us know what you have installed.
Also check that your device is updated. Connect the cover and run Windows update.