Found this article, though I would share it.

Direct from Vise Grips:

Attention All Bowler's.
 Our research has shown that after 40 to 60 games, the fit and texture of your finger inserts have changed. Finger inserts are made of a soft vinyl material, which wears with frequency of use similar to the tread on a tire or the sole of a shoe. Excess wear enlarges the size of the finger inserts holes altering the fit and span of your grip. Proper fit and span are among the most crucial elements of ensuring a strong sense of confidence and performance on the lanes. Additionally, finger inserts are exposed to various rosins, hand conditioners and lane oils which alter the texture and feel of the inserts. These substances absorb into the insert gradually changing and damaging its texture affecting the way the fingers or thumb exit the ball.

To maximize your performance on the lanes and keep the new ball in optimal striking condition, it is critical to properly maintain your bowling equipment. Visit your pro shop professional after every 40 to 60 games to replace your old inserts for optimal scoring every time.