50 years

 
  It seems that when God was making the world, He called Man over & bestowed
  upon him 20 years of normal sex life. Man was horrified! "Only 20 years of
  normal sex life?" but the Lord was adament that that was all man could
  have.
 
  Then the Lord called the monkey & gave him 20 years. "But I don't need 20
  years," he protested. "Ten is plenty for me". Man spoke up eagerly, "Can I
  have the other ten?" The monkey graciously agreed.
 
  Then the Lord called the lion & gave him 20 years. And the lion, like the
  monkey, wanted only ten. Again Man spoke up, "Can I have the other ten?"
  And the lion graciously agreed.
 
  Then came the donkey & he was given 20 years, but like the other animals
  before him, he felt that ten years was sufficient. And once again Man
  asked for and received the remaining 10.
 
  Which explains why man has

         20 years of normal sex life-
         10 years of monkeying around -
         10 years of lion about it -
                     AND
         10 years of making an ass of himself.