The performance of a golf ball is related to whether or not it is out-of-round and it's compression rating. You can hit the "sweet spot", but if your ball is out-of-round, it won't fly straight. 

Performance balls offer greater spin but also are made of very soft materials -- a gum derivative called balata has been the industry standard for years. These combine to give optimum trajectory characteristics with a high rate of spin. The downside? They tend to be so soft they can come out of round, or their covers can be cut, rendering their characteristics ineffective (excerpted from "Golf Balls: Sorting Out What's Right for You", by Ted Johnson, Equipment Columnist for GolfWeb). 

As many as 30% of off the shelf name brand balls will not roll true. Despite excellent quality control many mass produced balls are either out of round or the center of gravity is misplaced (excerpted from Purrfect Putters, now off-line). 

How long you can play the same ball before you start losing distance depends on who's hitting it. A good player could use the same ball for many rounds without noticing a loss of distance, but probably wouldn't, because cosmetically, the ball would lose its shine. A poor player, who swings the club at the ball like a logger swinging an ax, will probably lose distance quickly because the ball will be knocked out of round and won't have a true ball flight (excerpted from "Everything You Need to Know About Balls", by Jon Ebert of Scripps Howard News Service, hosted by GolfAtHalfPrice.com). 

Most pros do use several balls during a round because they play balata covered golf balls which cut much easier as well as come out of round. With the two-piece ball that you are playing, it should be playable as long as you have it. In time, the golf ball will lose compression which results in a loss of distance but it takes a long time (Mike McGetrick, Meridian Golf Learning Center, Denver, excerpted from USA Today's PGA Lesson Tee). 

Wound golf balls tend to go out of round during play as the windings lose their amalgamation with the cover and result in the distortion of the shape of the ball.  However, two piece golf balls, due to their construction, rarely go out of round. While it is difficult to visually notice a golf ball that is out of round, it does have a significant effect on the golf ball's flight and putting performance (excerpted from Wilson Sport's Golf FAQ). 

Question: How many rounds is a golf ball good for? I play a two-piece, 90 compression. Answer: Most pros use several balls during a round because they play a balata covered golf ball which cuts much easier as well comes out of round. With the two-piece ball that you are playing, it should be playable as long as you have it. In time, the golf ball will lose compression which results in a loss of distance but it takes a long time (excerpted from PGA.com's "This Week's Professional" question and answer instruction, by Mike McGetrick).