When using head and shoulders shampoo to get rid of dandruff and to keep your hair clean you should leave head and shoulders shampoo in your hair for 20 to 30 minutes.
Although some people only leave the head and shoulders shampoo in their hair for 2 to 10 minutes which is okay too depending on how severe your dandruff problem is.
Make sure to scrub the head and shoulders shampoo into your hair to remove the dandruff and then rinse it out after 20 to 30 minutes.
You may need to wash your hair more often when you have dandruff although it can be hard to completely get rid of dandruff.
But you can at least get rid of some of it and keep the worst of the dandruff out of your hair.
To keep the dandruff away you must wash your hair often with the dandruff shampoo otherwise the dandruff will eventually start to come back.