The Logitech H800 headset can take up to 3-4 hours to fully charge. It has a 6-hour battery life when fully charged.
When headphones are not checked out, they should remain plugged in to the charging station. The battery will not be significantly damaged if it stays plugged in after it is fully charged! (The headphones also do not have a "full battery" indicator, nor is there a way to see how much charge is remaining, so this is the best way to ensure that students don't get headphones with low battery life.)