Mouthguards vary in type, brand, material, and durability, so some may last longer than others. Regardless, always replace your guard immediately if:
If you have a habit of chewing your athletic mouthguard, it could be an indication of an uncomfortable mouthguard. In which case, you should look into wearing a custom fitted guard that doesn't inhibit breathing.
You can increase the lifespan of your mouthguard by taking care of it. Mouth guard maintenance is something that most athletes forget about, but it's actually an important part of your routine. Bacteria love the dark and moist environment of your guard, which is why you should sanitize it after every use. There's many ways to clean your mouth guard. The simplest way is with some good ol' soap and water. When you're done, dry it then store it in a case.
How does that ancient saying go? "Take care of your mouthguard and your mouthguard will take care of you." That's right. With regular cleaning and care, your athletic mouthguard will prevent painful and costly dental injuries down the road.