The different experiences that people have with battery life on the Kindle has led to a bunch of theories:
1. that turning the Kindle off saves more battery than sleep mode
2. that “indexing” of the books on your Kindle goes on in the background and drains the battery
3. that playing music on the Kindle drains battery life more quickly than reading
What do we know? First, sleep mode and “off” mode drain precious little power. I can’t tell the difference, and I don’t have to wait for the Kindle to boot when I want to start reading, so I go with sleep mode. Second, it seems like the phantom “indexing” comes into play the more “junk” you have loaded onto your Kindle, particularly if you are using an SD card for additional storage. It makes sense that the more file management you require, the more the battery will drain. Finally, who plays music on the Kindle?
My experience, with lots of sleep mode every day and no SD card, is that I charge the Kindle twice a week. Since most of my little freeze-ups and other funny outputs seem to come with a single bar showing, I charge the minute the third bar disappears.
What you can count on: me forgetting to turn the whispernet off after downloading the Journal every morning noticeably reduces battery life.