In general, children and teenagers should have their sports mouthguards changed every year to take into account growth spurts and changing dentition. Adults may keep wearing the same mouthguard until it starts feeling loose, uncomfortable or too tight.
Inspect your mouthguard often to check for signs of degradation and distortion, as these compromise the level of protection and will require replacement as soon as possible. If you're unsure, bring your mouthguard to the dentist for assessment.