Determining when to replace a golf ball can depend on a few factors:

1. Visible Damage: The most obvious sign that a golf ball needs to be replaced is visible damage. This can include cuts, scuffs, or cracks in the cover. If a golf ball has any of these signs of damage, it's time to replace it, as these damages can significantly affect the ball's flight and overall performance.

2. Performance Changes: If you notice that your golf ball is not performing as it usually does, it may be time for a replacement. This can include changes in distance, trajectory, spin, or feel. These changes can be subtle, but if you're familiar with how your golf balls typically perform, you might notice when things start to deviate.

3. After a Certain Number of Holes: Some golfers choose to replace their golf balls after playing a certain number of holes with them, regardless of whether they show visible signs of wear. For example, some may choose to use a new ball every round, while others may use the same ball for several rounds. This often depends on the golfer's preference, skill level, and budget.

4. Lost Balls: Naturally, if you lose a golf ball while playing, you will need to replace it.

Remember, even minor nicks or scuffs can potentially alter a ball's flight characteristics, and a deep cut could affect the core and significantly change the ball's behavior. When in doubt, it might be best to replace the ball to ensure consistent performance.