You should clean your brush at least once a week to avoid bacteria building up. Use a gentle baby shampoo or body wash to clean it.
As long as you keep your brush clean, you can continue to use it until you notice bristles falling out or any build-up that you can't remove.
Also, if the bristles are getting stiffer than you'd like, it's time to replace it. Make sure you're also replacing your razors and toothbrushes regularly to keep them working at their best.