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.
