You should replace your toothbrush at least every 3–4 months. And if you’ve only had your toothbrush for 1 month, and you have a cold or sinus infection, throw it away. There is no reason to re-infect yourself from using that toothbrush. Throwing your toothbrush out after 3 months seems like a short time, but think of the germs, viruses and bacteria it is exposed to daily.
When bristles become frayed and worn with use, cleaning effectiveness will decrease. Toothbrushes will wear out more rapidly depending on factors unique to each patient. Check brushes often for this type of wear and replace them more frequently if needed.