As someone else here said, 10 to 15 min... The MAIN thing is NOT to apply it around the eye area!! Your face should be covered, except for the eye area; if you do it right, you will look like you have 'eyes' for sunglasses!! A very good mask is MILK OF MAGNESIA... Yes, you read it right. Just get the store brand; same ingredients, lower price... Smooth it on your just-washed, face after you have dried it... Leave on for 10 minutes then rinse rinse rinse with COOL NOT COLD water... Blow dry and continue with whatever you do after... (moisturizer, etc.)
Around 10-15 minutes is enough. If you place it on your face more than 15minutes, the moisturizers in and on your face will decrease.