If you have dry skin or a dry scalp then you should put oil in your hair up to 3 times per week but if the scalp and skin is not very dry then putting the oil in your hair once per week is usually enough.
I have dry scalp and dry skin so I put the Lavender Oil in my hair 3 times per week which helps keep the skin and scalp from drying out and helps promote the growth of my hair.
I also find that my hair is thicker and shinier as a result of using Lavender oil in my hair 3 times per week.