I am currently using the UR30 model and testing high-torque screw tightening. I applied 150Nm and 220Nm torque in some cases, and there were no errors or position loss. I’m wondering how long I can apply this torque in screw tightening mode. Is the mechanical brake being applied, and if so, how strong is it?
If it’s working as it should, and you’re using the screw feature, then you should be good to go without worrying about extra wear on the robot. 
It is not using the mechanical brake for this, but simply stopping the motor by energizing it.
I agree with what @efn states and would add that as long as you are not getting protective stops you should be ok at these high torques.