When washing your hair with regular shampoo you should leave the shampoo in your hair for 3 to 5 minutes.
During that 3 to 5 minutes continue massaging the shampoo through your hair and work the shampoo in and then after 3 to 5 minutes of working the shampoo around in your hair you can wash it out.
It's not good to leave the shampoo in your hair longer than 6 or 10 minutes before washing it out because it could cause your hair and scalp to dry out and cause itching.
So make sure to wash the shampoo out of your hair within 10 minutes of putting the shampoo in your hair.
With dandruff shampoo you have to leave the dandruff shampoo in your hair for a minimum of 3 to 5 minutes for the dandruff shampoo to be effective.
So while you do not want to leave the shampoo in your hair too long you also do not want to leave the shampoo in your hair too long either.