Around 2-3 hours. During menstruation, it is recommended that sanitary napkins should be changed every 2-3 hours, especially when the temperature is high. Because sanitary napkins do not breathe well, if you do not change them in time, you will leave your perineum in a damp state for a long time, which will stimulate itching, redness and swelling of the skin. In addition, bacteria can easily grow over time, which increases the risk of gynaecological diseases. Not changing sanitary napkins for a long time may also lead to the overflow of menstrual blood, which can cause contamination of clothing.