Method for scheduling a refilling of a secondary reservoir of an ambulatory infusion system out of a larger primary reservoir of the ambulatory infusion system storing a liquid drug, the ambulatory infusion system further including a valve unit for alternatively coupling the secondary reservoir to the primary reservoir in a refilling state or an infusion site in a dosing state, the method including repeatedly and automatically carrying out a filling volume assessment routine, the filling volume assessment routine including: - determining, at a present point in time, an estimated filling volume of the secondary reservoir at a future estimation point in time, the future point in time being an estimation time interval after the present point in time, wherein the estimated filling level is determined by subtracting an infusion estimator, the infusion estimator being an estimator for the amount of drug that is expected to be infused in the estimation time interval, from the filling volume of the secondary reservoir at the present point in time, - and determining in dependence of the estimated filling volume, if the secondary reservoir shall be refilled at the present point in time, - the method including determining that the secondary reservoir shall be refilled if the estimated filling volume is below a filling volume threshold. wherein determining the estimated filling volume is based, at least in part, on an expected amount of on-demand bolus infusion in the time interval between the present point in time and the estimation point in time, and/or on a history of actual past infusion. the method including repeatedly and automatically carrying out a filling volume assessment routine, the filling volume assessment routine including: - determining, at a present point in time, an estimated filling volume of the secondary reservoir at a future estimation point in time, the future point in time being an estimation time interval after the present point in time, wherein the estimated filling level is determined by subtracting an infusion estimator, the infusion estimator being an estimator for the amount of drug that is expected to be infused in the estimation time interval, from the filling volume of the secondary reservoir at the present point in time, - and determining in dependence of the estimated filling volume, if the secondary reservoir shall be refilled at the present point in time, - the method including determining that the secondary reservoir shall be refilled if the estimated filling volume is below a filling volume threshold. - determining, at a present point in time, an estimated filling volume of the secondary reservoir at a future estimation point in time, the future point in time being an estimation time interval after the present point in time, wherein the estimated filling level is determined by subtracting an infusion estimator, the infusion estimator being an estimator for the amount of drug that is expected to be infused in the estimation time interval, from the filling volume of the secondary reservoir at the present point in time, - and determining in dependence of the estimated filling volume, if the secondary reservoir shall be refilled at the present point in time, - the method including determining that the secondary reservoir shall be refilled if the estimated filling volume is below a filling volume threshold. wherein determining the estimated filling volume is based, at least in part, on an expected amount of on-demand bolus infusion in the time interval between the present point in time and the estimation point in time, and/or on a history of actual past infusion.