I can say that I manage a day's use out of it, as in I leave for school at 9am, and when I get back at 8pm my phone is around 20%. However, I often use my phone for web browsing, especially in direct sunlight while waiting for the bus on campus (Average 30-40 minutes per day of browsing at max brightness), play games if I get bored (30 mins or so), check random facts and formulas as needed (10 mins or so), and use my phone to make about 2-3 quick calls a day. It really depends on how often you utilize the screen and at what brightness setting. If I turn the brightness down I could get almost 7 hours of constant use out of it instead of 10 hours at moderate use (Battery Use stats say I had the phone display on for 2.5 hours and was responsible for almost half the battery drain!). It all depends on how you plan to use the phone. An iPhone 4S will have a smaller screen and better battery life as a result, so if you use it a lot then you will definitely notice a difference.
For example, today:
10 hours 20 minutes on battery
40% display (2hr 35m)
36% Android OS
6% Voice Calls (16m)
6% Android System
4% Dolphin Browser HD
4% Cell Standby [Time without signal 26%(while I was at work most likely)]
2% Phone Idle
2% Robo Defense
As you can see, the display eats up the most battery, with the OS CPU needs nipping at it's heels. The CPU drain is to be expected as it is a dual core phone, but the battery life is still better than my captivate.