A simple model is presented which allows the profile of open flux within the magnetotail lobes to be determined from a knowledge of the present size of the polar cap and the past history of low-latitude and high-latitude dayside reconnection. The model shows that the nominal length of the tail is dependent on the elapsed time since reconnection of the oldest open field lines and that a much longer disconnected tail and wake can exist beyond this. The model is applied to two 8-hour intervals of observations, which indicate that the length of the tail can vary by almost a factor of 10, between ~400 and 4000 RE, in just a few hours. In addition, differing rates of lobe reconnection in the Northern and Southern Hemispheres are shown to lead to considerable differences in the lengths of the two magnetotail lobes.