While there are plenty of reports of readers getting up to a month of battery life out of the Amazon Kindle 3G, my experience is that the usage is about a week – less if you are reading a lot.
I have the Wifi/3G 6 inch EInk display model. I use it for about an hour a day. I leave it turned on with the 3G also on. This is a deliberate choice because I mostly use the Kindle to read newspapers.
The great advantage is that Whispernet sends the papers to the Kindle every morning so that they are already there when I board the train for my one-hour commute to the office. This is important because the 3G signal disappears for the first 20 minutes of the journey so if you forget to turn on and download the papers before the train leaves the station, you might just have to do without them for half the journey.
Typically, I estimate that I use the Amazon Kindle for about 5 or 6 hours of reading during the week. I charge it by connecting it to a laptop on Sundays so that it is fully loaded for the week ahead. It takes a couple of hours to fully charge using the USB lead. On a couple of occasions when If I forget to charge it on Sunday, it was dead on Monday morning so – regardless of whether it is in my bag untouched for the week – or used every day for reading – the battery only seems to last the week. That is something to keep in mind if you are a heavier reader than I am. I regret not purchasing a power charger because it would give me more options on topping up the battery during the week. If you’re purchasing one and they still have the offer on packaging in a charger, it would be worth getting one.