You didn't mention whether you have the Wi-Fi or the 3G model. If you have the 3G model, leave wireless on, and are not connected to a wireless network, your battery life will be significantly reduced. This is because the 3G takes more power than Wi-Fi and 3G gets turned off when you are connected to a wireless network. The best battery life can be achieved by turning wireless off when not using it.
Amazon states the following battery life:
- Kindle Wi-Fi and Kindle 3G, wireless off: 1 month
- Kindle Wi-Fi, wireless on: 3 weeks
- Kindle 3G, wireless on: 10 days
At any rate the battery life that you have experienced seems pretty short. If it doesn't improve after three charge sessions I would contact Kindle CS.