Depends on the day of your period and the type of tampoons you use. For the first day when blood flow is usually light, it is recommend you use a light tampoon. Change at least 4-5 hours once. When the blood flow gets heavier on the second day onwards, change to super tampoons and make sure you change regularly at least 3-4 hours. Towards the end of m, change to mini tampoon and still make sure you change every 4-5 hours.
Therefore is no hard and fast rule on the number of times you change your tampoons during m as each body is different but the rule of thumb is - never leave a used tampoon in your body for more than 7 hours as this induce baterial growth.