how fast does a sling shot need to be for the rock to be stuck into the skull?

Let's talk about terms here. I assume you're referring to the Biblical "David" story. (or myth, as you prefer)
Note that David did not have a "slingshot". No rubber back then, right?
David used a "sling", a prominent weapon of war for over 1000 years.
As a person with some slinging experience, I can testify that a skilled slinger can impart serious velocity to the projectile.

Although David is supposed to have used a rock, the standard ammunition for purposes of warfare in the ancient world was made of lead. What the Romans called "glandes"; a football-shaped chunk of lead weighing a bit over an ounce.

Note also (depending which version of the story you like) that David is not reported as having actually penetrated Goliath's skull. In fact, one account is to the effect that Goliath was merely stunned by the blow, and David subsequently cut his head off....

For all you would care to learn about slings and slinging history, I recommend:

http://www.slinging.org/

oic
yes i was refering to the Bible, David
thank you very much

i dont think the speed of the rock that matters. its the size of the rock an the kind of rubber you have (mines made of super strechy surgical tube, lots of power) :-D