is it the 4s? loads of people have said the battery is not as good as old ones.
It does depend on how much you are using it and whats running in the background as well though
well if its online its still using battery, youd be surprised how much it uses. I can take my phone away and use it for just occasional texts and it lasts 5-6 days. If Im on mobile facebook it drops by 1% about every 2 minutes.
I usually get around 2-3 days on a single charge with my HTC Desire. Given light to moderate use each day, I think that's pretty decent.
The main factor I find is 3G data connection. If you turn off the data connection and only turn it on when you're using data (email, online apps, web) you save loads of juice. The standby time will be significantly longer if the phone sits on the 2G band instead of 3G.
my iphone lasts about the same as mobius's htc, you may want to check you have not got a location checker on, another cause can be your mobile network losing its signal, the phone will keep searching for one!
I've got a 3GS and I found that I needed to charge it every other day but since I've had the iPad (since last Tuesday) I've only charged the phone once.
Have you downloaded the latest iOS update? My 3GS was dropping really quickly but since I downloaded the update it lasts a bit longer - but I check emails and stuff all day long and two days is still about the max unless you turn off the 3G and locational services.