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Set a timer and wait. Repeat the process once per week.

Article:
The length of time you will leave on your mask will vary based on the type of mask you are using, and the type of skin you have. In general, 15 minutes is a good average for most masks. Set a timer for yourself and relax.  If using a clay mask, do not wait for the clay to dry. Remove the mask while the clay is still slightly wet.  If using a store-bought mask, read the instructions to know how long to leave it on. It is important to avoid using clay masks too often because it can irritate your skin. However, using a clay mask can be a refreshing treatment for your skin. Use a clay mask only once per week to reduce the chances of irritation.   If your skin is dry, then you may want to use a clay mask less often. If your skin is oily, then you might be able to use the treatment more often.