How long with a battery for a Nikon D3000 last?

Question for my friend who will be doing a wedding from 9am-10pm

He has 2 batteries for the camera but planning to get more just in case. Is one more okay or should it be two more>?

Well it really depends on the amount of use and the features that you are using. For example, what drains battery a lot is using the flash. I use an external flash (Nikon sb 800) so I don't have to use the one on the camera. Another thing that drains battery is the LCD screen. I usually set mines very low (somewhere between -1 to -3). But if you have two fully charge batteries, you should be good. They last pretty long.