I find that my N80 can last me about two days before I decide to charge it. Battery still shows about two bars.
This is with moderate usage, few txt msgs, no mp3, browsing internet via wifi for about 30min/day, calls for about 40min.
So far, I haven't had a need to charge it the same day. Nevertheless, your mileage will vary.
Also, set in Wifi options not to report for any wifi.
And set to search for GSM networks only.