There is provided an electrically operated aerosol generating system (100) for receiving an aerosol forming substrate (115). The system comprises a liquid storage portion (113) for storing liquid aerosol forming substrate an electric heater (119) comprising at least one heating element for heating the liquid aerosol forming substrate and electric circuitry (109) configured to monitor activation of the electric heater and estimate an amount of liquid aerosol forming substrate remaining in the liquid storage portion based on the monitored activation. There is also provided a method in an electrically operated aerosol generating system comprising a liquid storage portion for storing liquid aerosol forming substrate and an electric heater comprising at least one heating element for heating the liquid aerosol forming substrate the method comprising: monitoring activation of the electric heater and estimating an amount of liquid aerosol forming substrate remaining in the liquid storage portion based on the monitored activation.
