I had the opportunity to use my Kindle DXG as my sole internet link during a trip to New York.
Charged the kindle the night before, 100%
Morning: spent about 4 hours reading(I read quickly, so one page flip maybe every 15-20s). 3G was on the whole time, time split between reading and surfing net(mostly gmail), maybe 50/50
Evening: spent about 6 hours reading(blame amtrak for delays lol). 3G was on the whole time, about 4 hours on the net surfing(reddit... was bored), 2 hours reading
Battery low warning came up near the end
Not bad... a little over 10 hours of web surfing and reading combined. Definitely loved it... took the boredom of the amtrak delay