After some deliberation I’ve come to the conclusion that you must keep oil in your hair for just 2 hours, no more and no less.
Your hair oil consists of ingredients like base carrier oils, traces of minerals, herbs, essential oils and vitamins. It takes two hours for these to get to your hair roots and in to your blood stream. So you need to keep oil on your scalp for a minimum of two hours.
Oil on your scalp cools the brain. Once the brain cools it works slower thus slowing down the functions of your entire body. So don’t keep oil on for longer than is required.
I will add that the best time to apply oil on your head is in the hotter part of the day. It is preferable not to eat a meal with oil on your head as your metabolism is slower than normal. So if you cannot keep the oil on from 11am-1pm, wash it off and eat your lunch, apply it one hour after you have eaten.