Well since using the battery App and using overnight charges, my iPhone 4 lasts a day. I unplug it at 8am ish and it usually goes back on charge around 1am ish. By 1am it's very low under 10%. I would say my usage is fairly high, but of course there are large periods during the day where I don't use it.

I heard that there were people who were getting 2 days out of theirs, I imagine they are just on standby with minimal use?