It varies. Check the Times Guide and New Information sheet which is next to the maps when you enter.
When we were there in Jan it was only held on Sun, Mon, Tues, Wed and Sat at 11, 12, 1, 2, 4, and 5. I am sure in the peak season they offer this daily and more often.
It was fun - we really enjoyed watching it. My DS (7) is extremely shy and he was hiding behind my niece (8) so that Merlin wouldn't pick him to be the one to pull the sword. It was so funny to see - since all hte other kids are dying to be the one chosen and there he was working so hard NOT to be picked.
Enjoy.