Just did a 30 minute test on my iPhone 4, started at exactly 71% battery, after 30 minutes it was 61% - on what I consider a normal brightness (this unfortunately is the biggest factor in battery life, so while it's nice to have stats with it maxed, I feel my normal usage is not accurately represented that way).
So extrapolating, 30 mins = 10%, a full charge would give around 5 hours of game time, which for me is not a big deal, anything over 3 is most likely more than I'd ever be doing at one go. Obviously it's pretty poor compared to handheld systems, but since cell phones come out with new generations every year, I'm not worried. In a couple years time, our batteries will last twice as long, and in another 5+ years, probably 5x as long.