I leave WiFi on all the time on my KIndle Kbd WiFi 3rd Gen. I get about 30 days of battery life with use of 30 to 60 minutes per day of reading.
However, since I've had mine from 8-Sep-2010, I've only charged it 26 times. I believe Lithium batteries only last for 500 full charges per the instruction manual for my NowBible Lithium battery.
If you download lots of books requiring extensive indexing, then the indexling, itself, draws power excessively. When I've done that, I leave the Kindle connected to the power so it has no resultant drain from the battery while the indexing is in process usually overnight.
Bibles, encyclopedias and dictionaries will require intensive indexing.
I am not sure but its possible usage of Caliber may use more juice than simple downloading from the Amazon Kindle Book Store. If that is true, it may pay to leave the Kindle connected to the power overnight while your Caliber downloading is happening.
Last edited by sirmaru; 09-26-2012 at 08:26 AM.