How often should you change your toothbrush?

It's not that the bristles looking good is what matters most - the reality is that five months is already way out of range. The recommendation is to change it every three months or so, four months max if you take really good care of it. By that point the bristles lose their stiffness even if they look decent visually, and the cleaning effectiveness drops quite a bit. It's also true that they accumulate bacteria, fungi, and toothpaste residue that gets trapped in the base, especially if you leave it in a humid environment like a bathroom.

The best thing to do is just throw it out and get a new one. It's not a huge expense and your mouth will thank you. If you want to make it easier, mark on your calendar the date when you change it, so you don't have to spend time guessing. Some electric toothbrushes even notify you when it's time to change the head, but with a manual one the golden rule is every three months or so.