Like stated, take it slow and easy. Don't let the metal of the hex "blue up", or if you see it start to turn blue, remove it and let it cool down.
Actually, the heat generated, if you are holding the allen wrench with your hands should definitely let you know you have held it to the grinder a tad too long.
These make great tension wrenches and, frankly, the weight of the handle in some cases is just the right weight to turn the plug.
Several people use these and pick "one-handed." They let the tension wrench do the turning work with no human assistance.