How much time does a Nikon d3100 need to charge for the first time?

I have just bought a Nikon d3100, but I don't know how much I time I have to charge it for the first time?

Actually you will be charging the battery using the battery charger that came with your camera.

Usually it takes less than 4 hours when the battery is flat. In most cases, just charge the battery the night before you plan on using the camera. The charger will automatically shut off once the battery is fully charged. When the light on the charger stops flashing, the battery is fully charged

Yes, since you're not charging the camera but it's battery just watch the light on the charger.