Zero. I don't see the point.
My scalp creates its own oil and my hair does not need it.
i dont see the point either... unless it's a tiny amount to assist in scalp massage
2-3 times a week. Mostly with grease and sometimes with Taliah Waajid Healing Oyl (that's nice and thick too). My scalp is really, really dry and flakes when it doesn't get some kind of moisture. I do wash my hair (or braids) weekly, but that alone isn't enough.
I oil my scalp when it feels dry. But I generally don't have to go out of my way to oil it because my conditioners have such a high oil content (AO).