The Galaxy S III has a capacity of 2100 milliamp hours. As far as battery life goes, it's alright, when compared to other flagship phones from last year. But if you want the absolute best longevity out there, not to mention the best bang for your battery buck, you've got to check out the off-brand expanded model that Reddit found while trawling the depths of Amazon.
It's got a capacity of 7000mAh. Look at that number again: 7000. That's more than three times the capacity of the standard battery, double most of the existing extended batteries, and (as it happens) the same amount of juice that goes into Samsung's current crop of 10-inch tablets.