From my tests with my N900 I've seen it chew up 700mA for short durations but fairly easy to make it draw ~500mA for relatively long stretches. I would guess that a good, standard 1.3AH BL5J battery should last around 1.5 hours minimal for the worst case usage from fully charged to flat.

I think the worst case "real" usage pattern is having a speakerphone phone call while having backlight on full blast and having it constantly route packets through 3G using ad-hoc wifi... Sound about right? Or maybe you can be playing a game at the same time

The battery shouldn't get warm when charging. This is a bad sign. The battery compartment may get warm during discharge but likely this is the GSM module or CPU sharing the heat... That 2000mAh cell may have high internal resistance and probably should be ...tossed...