Reposting what I said in another thread on this topic + an updated note:
I haven't been overly scientific about my battery usage but I read everyday for 1-2hrs a day. Only sleep my Kobo Glo and use the Glo function 1/4-1/3 of the time and it'll last over 30-35hrs on a single charge might stretch to 40hrs in some cases.
So it'll go about a month using sleep mode while reading at 2-4x the usage rate of Kindle's* battery usage amounts before I have to recharge.
Wifi off at all times (I never turn it on)
*Kindles #'s if I recall are 30min reading per day.
Additional Note: This charge cycle I've used the Glo function way less than previously and I'm already at 39hrs on this charge and still the battery indicator says I've got 55% of a charge left. (Though % drops like crazy towards the end of the cycle)
I only charge the battery when the thing tells me it's dead.