It's one of those questions that nobody really knows the answer to, but it's quite important that you do.
Studies show that toothbrushes are far less effective at removing plaque after the three-month period, meaning you should ditch the old one and fork out on a new one - or if you're an electric toothbrush fan then replace the head.
Celebrity dentist Dr Bill Dorfman suggests that it's also important to change your toothbrush after you're sick or have a mouth infection.
He adds: "Even if you haven't been sick, fungus and bacteria can develop in your toothbrush."
His final tip for dental hygiene success: make sure you let your toothbrush dry out between uses.
"Toothbrushes can be breeding grounds for germs, fungus and bacteria," he adds.
"After using your toothbrush, shake it vigorously under tap water and store it in an upright position so that it can air out."
Take Dr Dorfman's advice, follow the steps below and you'll be on your way to earning a Tom and Jerry sticker in no time...