If you have a manual toothbrush, try to adhere to the 3 month replacement rule or sooner if you had a cold or flu. Some toothbrush manufacturers have colored center bristles. When the color fades half way, its time to replace. Some electric toothbrushes (like Sonicare) have a built-in UV sanitizer that can extend the life of replacement heads. Also a good rule of thumb for any toothbrush is to keep it out in the open after use so it can dry completely between brushings.