Since the ditch dimensions are fairly consistent, a reasonable, if very approximate, estimate can be given. A volume of 1,465,313m³ has been calculated as the amount of material that had to be removed to form that ditch. It was then constructed into the north and south mounds along its 112km length. This would apparently have taken 949,667 man-days. Incidentally, since a cubic metre of topsoil with some moisture weighs around 1.44 tonnes, that suggests a minimum of 2,110,051 tonnes (only a minimum, as some of the ditch was excavated through rock, as at Limestone Corner).
Further reading: Hill 2006