It all depends on usage. The mentioned times were for total idle. On real usage you can probably drain the battery in a working day if you actively use the phone with battery draining tasks. But if you have low usage, you should be able to get real battery lifetime of 3-5 days. As for your screenshot, if you rebooted your N900 just after fully charging it, you must have either used it very much during that 1 hour and 28 minutes, or triggered battery drainage bug.