Some shampoos contain ingredients such as Biotin with the expectation that it will promote hair growth. Presumably the ingredient is in a format that can be taken up by the skin (or the hair follicles), or the shampoo contains some additional agent to support absorption.
With this question I am assuming that the absorption process would take some minimum amount of time (seconds or minutes), and that the shampoo would need stay on the scalp during that time in order for the ingredient to be absorbed.
However the labels on some of these shampoos do not provide any indication on how long the shampoo would need to stay on the scalp before rinsing it off.
The question I would like to ask:
Are there any guidelines how long this kind of shampoo should be left on the scalp before rinsing it off?