When you use purple shampoo for your hair to wash your hair you should leave the purple shampoo in your hair for a minimum of 15 minutes.
You should not leave the purple shampoo in your hair longer than 20 minutes though so after about 15 minutes of putting the purple shampoo in your hair it's best to wash the purple shampoo out of your hair.
If you have brassy or color-treated hair you don't want to leave the purple shampoo in your hair longer than 15 minutes because it can make your hair turn out bad.
Purple shampoo is not a hair dye so leaving the purple shampoo in your hair too long won't turn your hair purple but may cause slight lilac tint to your hair.
So don't leave the purple shampoo in your hair longer than 20 minutes.