Don’t forget to watch the clock when you’re oiling your hair. After all, we believe that you can do too much of a good thing. While it’s perfectly fine to leave oil in your hair anywhere from three to twelve hours overnight, it’s best to not overdo it. If you apply too much hair oil, or keep it on for too long, it will result in you having to spend extra time in the shower getting it off. Read: You’ll dry out your hair with excessive shampoo. One to three pumps of oil should suffice for your hair, depending on its length.