- Till the number of days recommended by the dentist(generally 3 months)
- Till the number of days recommended by the toothbrush company(generally 3 to 6 months)
- Till after an illness
- Till the toothbrush just wears out
- Till the toothbrush starts stinking
- Till it gets lost
So a scientific method would be to have a few bristles that act as a Ph strip. The bristles will be white till the Ph value crosses a particular threshold value. After the Ph crosses the threshold value and starts to become a breading ground for either fungus or bacteria(acidic or basic) the bristles will change color. Few other chemical tests(such as tests for toxins) can also be used to change the color of the bristles. The user of the toothbrush can change the brush after the color changes.
Tooth brush change frequency can also be calculated based on Number of bristle breaks per day averaged over a 3 month period to get your individual toothbrush change frequency for any particular brand of toothbrush.
The toothbrush companies will probably introduce these type of toothbrush when they are economical and desirable to all.
Do comment below if you use some other criteria for changing your toothbrush.