I had a Nokia 6310i when I was younger, and on average (short calls, a few texts per day) the battery would last for 2 weeks. It had a small (but perfectly formed) black and white screen, a distinct lack of apps, and no 3G. All helps.

Interestingly, the claimed talk time, at 7 hours, was no better, and in fact possibly a bit worse, than a modern iPhone. Draw your own conclusions.