Never leave a tampon in for more than 8 hours when on your period. If you find you have to change your tampon more frequently than every four hours, you may need to use a tampon with a higher absorbency. On the other hand, if you change a tampon after wearing it for eight hours and find that it's still slightly dry or is uncomfortable to remove, you should switch to a tampon with a lower absorbency. Always use the lightest absorbency for your flow.
Changing a tampon too frequently can affect the tampon's ability to absorb menstrual flow -- that's because the tampon needs a little moisture to expand.