If the mat is covered with a thick fitted protector or blanket, or if you are lying on it, it will start heating immediately and reach its highest temperature in about 30 minutes. If the mat is left uncovered and the ambient temperature is low while the airflow is high, it may take the mat 1-2 hours to reach its maximum level. However, it is still possible that the mat may never reach the same level as it would if it were covered.