My original Nokia BP-4L is still good for 6+ hrs after almost 3 yrs of use. But my workday is 8+ hrs, so I bought a $3 Ebay special, and you get what you pay for. At full charge the thing has never lasted a second over 2hrs, and this is with screen brightness all the way down, and bluetooth & gps off, and only using the N810 for web browsing/skype via wifi at work.

Found this replacement battery on Amazon, it had good reviews and a reasonable price. Been using it for a few months now. For the first month or so it seemed as good as the original Nokia, but now I am only getting a solid 6+ hrs out of it; still, for $12.95 it is a decent replacement. So between my original Nokia, and this replacement, I am happy and have more than enough battery life to carry me even through overtime!