There is no hard and fast rule to how often you will change darts, but when you sense a problem, it's time you should do your research. Now, the problem may not be in the dart anyway, perhaps your dart flight needs replacement or maybe your dart point has faltered. If your darts are bouncing out more than normal, that's when you should really go after a solution, if you observe that your dart point is rough or too blunt, you could change the tip using a repointing tool.