If I don't really use my N900 at all it will usually last about 9 or 10 hours. I'm not connected to any IM or SIP but it does check e-mails.

If I use it for any reason then time is much shorter.

And sometimes it's really bad depending on application. For example, on Saturday I took phone off charger -- green light, fully charged -- used eCoach for 45 minutes, and 3 hours later my phone was giving me the low battery sound and warnings (i turned it off at that point). So less than 4 hours from fully charged to nearly dead battery.