The power consumption of the N810 with both GPS on and tracking and display on does use quite a bit of power. Expect about 4 hours.

Having charger connected helps, but it may not give you infinite time: I used N810 GPS with MaemoMapper couple months back. When it chimed the low battery alarm I plugged it into my cigarette lighter charger. It said charging, but still after about two hours it finally said "recharge battery" and shut down. (The charger was a cheap model from eBay, thus may be lower current rating than others... YMMV.)