While display brightness is one of the largest drains on battery life, how you're using your device plays a part as well. There's far too many variables that come into play when one asks such a broad question about battery life.
My real world experience with a large number of iOS devices & OS X laptops has proven over & over, that Apples one of the best when it comes to battery run times and recharge times. Finally I've yet to have any Apple battery fail with less than the estimated number of cycles. That's not to say it can't happen, but it's a rarity when it does.