I think 70k is the minimum although some of the 70k loads could be a bit longer on the pickup-to-dropoff part. Yet i didn't run into any troubles with the DM part during my tests and considering i earned more than 100k on some long yet easy to drive loads it's very easy money for my taste. Also not sure if i just didn't test long enough but i missed a dropoff point at Mt. Chiliad, because hauling up Chiliad is pretty much the Cirque de la Sarthe of SA-MP trucking imho.