Physical pain? Zero.
Now emotional pain? I'm not sure it would even be able to accomplish emotional pain. So much of our emotions are intertwined with chemical balances and releases. If a brain achieved consciousness, but had none of these chemicals at all......I don't know that'd even work.
Didn't have to. Kind of an obvious thing to point out, but OP didn't specify what type of pain he meant, so I figured I would, just in case.