what measurement of mm determines when to replce break pads?
About 1 to 2 mm lining thickness would cause me to replace brake pad. If you see one at 2 mm, there is likely another at 1 mm. You just want to avoid metal to metal.
Don’t know about break pads but Mr. Google says between 6.4 and 3.2 mm that brake pads should be replaced .
A good standard is when lining is thinner than backing plate, it’s time.