What do you mean?
Do you mean battery life/runtime?
iPhones don't have reserve/backup batteries (however that would be awesome)
Also, battery life is relative depending on what cellular technology being used.
The average user in Europe can plan for the iPhone 4S battery to last about this long.
3G can run for 10 hours
2G all day
Low-bandwidth is key here, viewing mobile websites is very slow. Anything else for short-term communications should be okay.
Browsing is so slow on 2G that the amount of constant communication the phone has to do just to load a single website is that any battery gains will not be seen with always-EDGE.
Chat? As in a chatting application? Chat as in a voice call? Chat as in SMS/iMessage?
Here's what I would recommend:
Turn off 3G when you don't need to use the phone for long periods of time.