I always keep my Kindle 2 in sleep mode. I am constantly picking it up to continue reading and if it was in off, it would take to much to turn it back on. Like others, the only time I use Whispernet is to download and I am now in the habit of turning that off after the book(s) download. With that said, I am seeing 2-2 1/2 weeks of battery time.
Now I have heard from others that they do turn off their Kindles when going through airport security. No one knows for sure if that is the right thing to do or not, but most indicate they do this as a precaution before sending the Kindle through x-ray. Hope this helps!