Seems like a large number of customers are complaining about the iPhone 4s battery life:
Christina Bonnington via Wired.com
Although Apple has not yet officially commented on the issue, according to The Guardian, some of those affected by the issue have been contacted by Apple’s engineers. One individual said that Apple called and, after asking a number of questions about his usage habits, asked him to install a monitoring program so that they could better diagnose the issue.
The iPhone 4S has a built-in rechargeable lithium-ion battery. It’s supposed to provide up to 8 hours of 3G talk time (14 hours of 2G) and standby time of up to 200 hours. Although it’s a slight larger (capacity-wise) battery than that of the iPhone 4, the iPhone 4 is supposed to get 300 hours of standby time. For more intensive activities like internet use and watching videos, on the iPhone 4S you get 6 hours on 3G and 9 hours on WiFi, and 10 hours, respectively. The iPhone 4S has a beefed up A5 processor and several other hardware upgrades and changes compared to its predecessor. When the iPhone 3GS debuted, it also suffered from battery life complaints.
I bet this will turn out to be an iOS 5 issue, somehow related to apps running in the background.
Update: Oliver Haslam reports that the ‘setting time zone’ function can drain the battery when turned on.