I think the easiest way to describe N900 battery drain is like this:
- If you use it as a phone - you should get average phone battery life (~4-5 days).
- if you use it as notebook - you should get average notebook battery life (~6-8 hrs).

I bought mine about week ago and still experimenting. I did PR 1.2 -> 1.3 when the phone was "clean". I noticed that in 2G mode and all remaining RXs and TXs off it uses about 8% every 10hrs. Which in theory should give about 5 days. On the other hand I can easily drain 20% in 1hr watching internet TV or high definition film.

I have standard 1.3 kernel. Brightness 2/5. I don't use sdcard.