Less than 20Nm I suspect, that would be enough to break the wiper if there was an obstacle.
Here's a rough guide to torque for humans:
0.1Nm - finger and thumb twisting something small
1Nm - whole hand twisting, as with a screwdriver
10Nm - turning a steering wheel with both hands.
Any motor putting out 20Nm would easily cause injury too, I suspect wiper motors generate about
5Nm, less likely to snap the blade when icy.