A method for determining the degree of saturation of a solid ammonia storage material in a storage unit includes activating a heater to release ammonia from the storage material until the pressure of the storage unit reaches a predetermined pressure. The method then deactivates the heater and determining a decay rate of the pressure of the storage unit while the heater is deactivated. The method estimates the degree of saturation of the ammonia storage medium in response to the decay rate. According to some embodiments, determining the decay rate may include measuring the time required for the pressure of the storage unit to drop from a first pressure threshold to a second pressure threshold.