Yes, it's take about 4hours to fully charge on your iPad by using Original iPad Charger. Try beware of compatible charger (dont use iPhone Charger) because those charger below 2A (you can found this specification on your charger) will take longer time to charging your iPad.

By the way, for iPad 1st generation, it's recommended to buy iPad Battery Case that fit with iPad. It's now got 8000mAh series that can let us playing iPad continuously >24hours, and I wont get headache regards problem of out-of-battery since this case is fit to my iPad and I just need to plug USB attach to this casing while charging my iPad