According to the Centers for Disease Control and Prevention, you should replace your toothbrush every 3 to 4 months.
This app keeps track of this interval, so you don't have to.

Just input date of first use and forget about it. The app will notify you when the toothbrush should be replaced and will keep bothering you until you do so. The app will make it very hard to use the brush longer than you should.

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