I have a 1500mAh battery from an Imagio and will get about 26 hours out of it with wifi on all the time and moderate to heavy use at times. Exchange email running as items arrive, RSS feed updating every 30 minutes, Friendstream running every 30 minutes, blah, blah, blah.
I charge it all night (from about 11pm to 6am), leave it plugged up but power the device off, it charges for another 30 to 45 minutes. Take it off charger and I am good to go for 26 or so hours. Please note that I charge it most every night even though I still have 40% or so battery left... there are days (usually the weekend) that I do not charge it over night, so the 26 hour number is pretty realistic for me.
For what it is worth, I got almost 36 hours out of it one weekend...