While conditioners and intensive treatments require time to soak into the hair, timing your shampoo is tricky. You want to get your hair thoroughly clean, but exposure to detergents can be a slippery slope. The answer to this question rests on the type of detergent used in your shampoo.
The 1-to-3-Minute Massage Rule
Overall, most shampoos don’t need to be left on for a long period. Shampoos work on the surface of the hair and scalp, without soaking in. A shampoo contains both hydrophobic and hydrophilic molecules. This formulation allows the product to bind to grime and oil so they can be rinsed away.
However, that doesn’t mean you just slap shampoo on and then rinse it off. You need to rub the shampoo into the scalp to enable those molecules to do their job. Ideally, you want to massage your shampoo in for anywhere from 1 to 3 minutes, depending on your hair's thickness, before rinsing.