The urn is filled when the player has earned 4,750 Cooking xp with the urn in inventory. Once the cooking urn is full, it becomes a cooking urn (full) and can be teleported to Ernie for 950 Cooking experience. The urn will give you 20% more xp if you cook with it. Beverages do not fill the cooking urns. You can only have a maximum of 10 urns at a time, whether it be banked or in your inventory. A message will show up in the chat saying "You should get rid of some of your cooking urns before starting a new one."