What yepp said: store it at around 50% in a room temperature environment, ideally. Also, you should NEVER leave something on a charger for that long as a) you'll generate heat when the battery starts to lose a bit of charge and the charger charges that from being inactive while being on and b) a battery was meant to be discharged and then charged again, which is known as a charging cycle.
What you want to do is use your battery in an efficient way and try to charge it to a point where you think that you have enough juice to get you through your day, and then charge it again once you don't think it's enough to repeat that. In other words, even if it's not low, topping off your battery once you get below a doable charge percentage would be best and then rinse and repeat. Better to charge up to or near full and then run it down to say, 50% before topping off again, than to run the battery down to empty and then charge it up to max again as this will account for one full cycle, which all batteries are rated for at a certain amount before degrading in their abilities to retain a charge!