“Forty” shows up in the Bible in such places as the 40 years in the wilderness for the Jews; 40 years from the crucifixion of Jesus to the destruction of Jerusalem; 40 days of Moses on the mount receiving the Law; Moses’ life being divided into three sections of 40 years; 40 days given to Nineveh; 40 days of Jesus being tempted by Satan in the wilderness; Jesus being with his disciples for 40 days following the resurrection; many of the Judges judged for 40 years; Saul and David reigned for 40 years; etc.
http://www.todayifoundout.com/index.php/2010/08/the-biblical-expression-40-days-and-40-nights-just-means-a-really-long-time/