The usual length of a day's journey in the East, on camel or horseback, in six or eight hours, is about 25 or 30 miles. The "three days' journey" mentioned in Exodus 3:18 is simply a journey which would occupy three days in going and returning.
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Bibliography InformationEaston, Matthew George. "Entry for Days journey". "Easton's Bible Dictionary". .