Here is an article that offers some tips for conserving your battery. Have a look.
http://us.blackberry.com/support/blackberry101/tip
If you try those tips and are still having issues with your battery, please get back to us. 
It could be either. Typically, an average smartphone battery will last 2 years so for it to be packin' it in after only 7 months seems odd. Usually, a Li-Ion battery, which is used in BlackBerry phones, will just konk out suddenly as opposed to slowly fade away, so I'm thinking maybe you have an issue with your phone.
One thing that will kill a battery is having many apps running at the same time. Press and hold your menu key for a second or two. There will display a list of apps that are currently running. Some apps, such as Home, Messages, Phone, Browser and maybe one or two others will always be running. If your list is much larger than that, consider shutting down some of them. To do this, go to each app individually, press the menu key and select Close or Exit. Pressing the red phone key to shut down an app doesn't do that, it just minimizes it to the background.
One more word of advice: any app that runs the phone's GPS will kill your battery quickly. Google Maps is notorious for this. For this reason, don't leave ANY GPS apps running unless you're using them.
I'm not sure what else to offer here. I've given you a couple of suggestions. I hope they help you.
