How long to bake natural clay?

Ok 2 days ago me and my friend were playing in a creek and we found a huge deposit of natural clay. So it was so heavy that we barely got home. I made a bowl and I don't have a kiln. I'm a minor by the way pre-teen I won't disclose what age. My oven only goes to 500 degrees. I wrapped my creation in tin-foil to keep the heat inside. How long does it need to bake?