Variable. More then a couple of mm but less than the width of a finger (i know someone who knows that firsthand (pun intended) from putting his fingers where they shouldn't be while equipment is running).
Ouch! I wish I could say I didn't know what that was like, but my thumb and cracker had a not so friendly encounter of the same type.
Thanks for the feedback, I assumed it was variable, but I am amazed at how many websites talk about "high shear" without actually defining "high."