I have been charging both of my iPad every night since the day that I got each one of them and I still get about 10 hours a day - every day - out of both devices. My original iPad is 22 months old and my daughter uses it every day. My iPad 3 is 10 months old and I use it every day all day long at work. I am using it right now.
Charge the battery when ever it needs to be charged or whenever it is convenient for you and don't worry about it.