Generally, use the battery for however long you need to. If you can try to charge it again when it gets around 50%. It's not a huge deal though, so don't get over-serious about that. Also, you should follow Apple's calibration instructions about once a month. A good rule to go by is try to use about 2 cycles on the battery every week. And, like already mentioned, don't keep the charger plugged in for extended periods of time. Give the battery some usage time or it'll lose capacity.