I bought mine back in March and used it very rarely, due to a broken speaker. I bought it well used with the battery life presumably reflecting this.

It gets so little use that most of the time it runs only on standby, albeit with GSM and Wifi on, so it receives the occasional email which I ignore since I rexceive the same on my main phone, my N900.

At this use pattern, only displaying the billboard standby screen, it lasts about a week on a single battery charge with just the stock battery. I see no reason why it should not last a month with an external battery pack and without WiFi and GSM.

Of course, if you mean anything other than the standby screen clock, the situation may be a bit different.