I just completed a test on my MakerPhone/Ringo with a 4G module connected to T-Mobile (via Ultra Mobile) UMTS network with a good signal but without making or receiving any calls, and receiving just one text message just to test the connectivity.
A full battery charge lasted 54 hours, with the display off (asleep) except for the occasional (6-8 times a day) wakeup to see if it was still going.
Contrast this with the same phone sitting in a room with marginal reception of the UMTS signal, so much so that it would frequently lose the connection and need to search for a signal to re-establish the connection again and again. In this scenario, the a full battery charge lasted less than a day.
If you are actively making/receiving calls and/or playing games (i.e. keeping the display backlight on), the battery will obviously discharge much faster.