This has been my battery-charging system:
1) On a day to day or week to week basis, I charge it when the battery gets to about 30% or so. I let it fully charge back up before using it again.
2) Roughly once a month, I let the battery run completely down, to say 2-3% and then I fully charge it. This helps to calibrate the battery meter so the readings are more accurate.
Of course, you can charge it when it gets to 40% or even 60% and you wont hurt anything. The batteries are pretty robust in these things from my experience. Basically, charge whenever you see fit. Once in a while, let it run down almost totally. Thats about it.