Life of a Golf Ball

Question
Assuming the ball is not lost, how long does a golf ball usually last?  Or, how often does a pro change the ball when playing in a tournament?

Answer
Life of the golf ball is impacted by various things.
1) The coarseness of the club face.
2) How the golfer comes into contact with the ball. Meaning if the golfer pinches the ball will the shorter the life span.
3) What material the manufacture decided to use on the cover.
4) Lastly and my personal favorite how many times the ball experiences something other than the golf course. For instance the ball may experience road rage when striking the cart path.

For premium golf balls and better amateur player's golf balls can last as long as round however I would expect the same golfer to go through no less than 2 a round.

As for the tour players, I don't have any real data. Although I suspect the average tour pro would go through no less than 4 a round.