I've so far never turned mine off. I haven't even put it to sleep mode. When finished or having a break in reading I just lay it away and let the screensaver come on when the Kindle decides itself.
I'm into my 2nd charge, and the very first charge lasted 26 days with Wi-Fi always on and never actively put to sleep og switching off. I've downloaded (via Wi-Fi) just less than 100 books and read 5-6 books on the initial charge.
Since I'm off with an airplane tomorrow morning, I've switched off the Wi-Fi for the first time now 1/2 an hour ago.