As little as possible! Most of the instructions for rehydrating coir blocks refer to their use in horticulture, but in an Air Head, you want to keep them as dry as possible. Some customers find that placing the coir brick in a ziplock bag and adding a small amount of water at a time works well. You can break the brick up, adding more water if needed. Warm to hot water (not scalding hot!) penetrates the coir quicker than cold water.