iPhone 3GS never goes into standby
Have you tried Settings > General > Auto Lock?

Now that you've clarified the question:

The reason why I asked if you were in an area of bad coverage/weak signal is if your signal is weak then your battery will drain quicker.  However most people I know with iPhones would only get 24 hours out of a full charge anyway.  I usually plug it in at night and unplug it when I get up in the morning and generally it will last all day.  If you find your battery is draining too quickly for your liking you can do some of the following.  

Turn off push notifications
Lower screen brightness
Turn off WiFi
Turn off 3G
Close apps which are running in the background  

I'm guessing you haven't had a smartphone before and are used to getting a few days from a full charge.  Smartphones use battery much more quickly than traditional cell phones because of larger screens, more powerful processors and generally more usage.