Battery life questions are sort of in the same vein as "when did you stop beating your wife?". There's no good answer, particularly when you're still tweaking the device and the answer may change up or down. Answer based on current development and you either create an expectation you can't deliver, or give a disappointing answer that will be improving. You can't win.
Originally Posted by MannyZ28
On a more profound level, IMO battery life is never good enough, because manufacturers tend to respond to market demand by adding features and eye candy until battery life is tolerable. To get better battery life, you'd have to throttle something back, and devices sell based on feature list and responsiveness during in-store trials. So you'll push power usage as far as you can get away with without making the battery life totally untenable.
Battery size is also a trade off. Make the battery bigger, and the device gets thicker and clunkier. Thick and clunky doesn't sell devices when you try it out for 5 minutes in the store.
Yes, some devices will have better battery life than others. And yes, long battery life is definitely a sales feature. But the size of the device, how that size feels in your hand, and your perception of responsiveness and "cool features" will close more sales. So we end up with smaller batteries that are just good enough to not be a problem. IMHO.