There are different factors to bear in mind when considering when to change your replacement grip, and they range from comfort to traction, down to rips (or even tears). When your grip is becoming slippery or you notice the racquet’s handle is getting too firm, you know it’s time for a change.
As a matter of fact, your grip is long overdue for replacement when it begins to tear or fall apart. You don’t have to wait till it gets to this stage.