The invention is directed towards a system for ambulatory drug infusion over an extended time period from a flexible container, the system comprising a filling apparatus with a hollow support structure (5,10) defining a container compartment (15), the container compartment (15) being adapted to receive, fully or in part, the flexible container the support structure (5,10) being adapted to limit the expansion of the flexible container (20) by contacting the flexible container (20) upon being filled, thus defining a maximum filling volume of the flexible container (20). The invention is further directed towards a container assembly comprising a flexible container and a support structure.