I recharge when it gets to around 20%.
I use it until it shuts off on me, then I charge it :3
Depends on what I need to do later. If I know I am going to be out and about later and will not have easy access to recharging I make sure it is fully charged before I leave no matter where it was when I check it earlier. If I am not going out and can recharge at anytime I wait for it to tell me it has a low battery, which is at 20%.