I usually have my Galaxy plugged into my laptop with the USB cable most work days, and hooked up to the charger every night. You might say I only need the battery to carry me through my spare time.
My usage varies quite a bit, mostly light entertainment (reading, sudoku, no intensive games). Every now and then, I need geo-awareness, for walking directions or exercise monitoring. I always use 3G telephony (there's no 2G coverage for me here), and sometimes have GPS enabled even if I don't use it at the time. Wifi is only on when I need it.
With this setup, I get one, maaaybe two days of normal use. The other day it ran down completely in under 24h even though it stayed in my pocket. (I still feel that nothing beats the Psion and Palm pda's of yesteryear.)