There is provided an electrically operated aerosol generating system for receiving an aerosol-forming substrate. The system includes a liquid storage portion for storing liquid aerosol-forming substrate, an electric heater including at least one heating element for heating the liquid aerosol-forming substrate, and electric circuitry 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 including a liquid storage portion for storing liquid aerosol-forming substrate and an electric heater including at least one heating element for heating the liquid aerosol-forming substrate, the method including 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.
