Do I have to wait the usual 28 days drying time until the first firing or can it be done earlier?

When the oven is insulated, it can be put into operation slowly and should be fired slowly for 2-3 days at a time with only 2-3 sheaths of wood, so that the moisture can escape. After the fire is out, close the baking chamber with the flap. After that the oven is ready for use!