When you say original, do you mean it is a genuine Apple Ipad charger? If not you may have bought an iPhone charger but even then it should be charging more than this in the time you have said.
When you say you leave it, I assume you're not using it? Do you have it on at all? I'm trying to find a reason it could be using charge. Try switching to another power outlet in case the one you are using is defective. If this makes no difference I would return the charger as it is either defective or only suitable for an iPhone or both.