Magnetic reconnection X-lines can vary considerably in length. At the Earth’s magnetopause, the length generally corresponds to the extent in local time. The extent has been probed by multi-spacecraft crossing the magnetopause, but the estimates have large uncertainties because of the assumption of a continuous X-line between spacecraft and the lack of information beyond areas of spacecraft coverage. The extent has also been inferred by radars as fast ionospheric flows moving anti-sunward across the open-closed field line boundary, but the physical connection of these flows to reconnection is not fully confirmed. We combine THEMIS spacecraft and SuperDARN radar measurements to reliably measure the X-line extent. We find that both spatially patchy (a few Re) and spatially continuous and extended reconnection (>10 Re) are possible forms of reconnection at the magnetopause. Interestingly, the extended reconnection develops from a patchy reconnection via spreading across local time. Such a spreading behavior is consistent with conceptual and numerical models, which predict that magnetic reconnection starts at a localized region and then spreads out of the reconnection plane. Different simulations have found the spreading to occur at different speeds such as the Alfvén speed and speed of the current carriers. We determine the magnetopause X-line spreading speeds and test with the model predictions.