Usually, for heavy flow days, it's recommended to change your tampon when its full or after a maximum of 8 hours. Also consider a super tampon for the extra absorbency. Alternatively you may like to consider talking to your GP about hormonal contraception like the pill or progesterone rod - this can lighten the flow and allow you to skip periods altogether. The general consensus for when a period stops is BMI (body mass index) less than 17. This of course varies from person to person. Your period stopping is a sign that 'mother nature' thinks that it would be unsafe for you to carry a baby. If you are fit and healthy then you would still be able to be pregnant even if you have a low body fat percentage, and so your periods would continue.