Why 40 days?

"And he said unto them, Go ye into all the world, and preach the gospel to every creature. he that believeth and is baptized shall be saved; but he that believeth not shall be damned. So then after the Lord had spoken unto them, he was received up into heaven, and sat on the right hand of God. And they went forth, and preached everywhere, the Lord working with them, " Mark 16: 15-16,19-20