...more than 8.5 hours (their listed battery time with continuous use). Simple. Leave it on the lowest Brightness setting and only turn it up when you need it. I use it at night and just when I don't want to be blinded by my screen, so it's not a big deal for me. I will brighten it for Youtube or the various games I have installed.
Using this tip, I managed to get my Fire to last two weeks - by accident, I couldn't find it!! It helps to be able to just grab it and go without having to remember to charge except every couple of days.