"As soon as it starts to hurt" is not some hopelessly complex and useless standard that involves delving into philosophies of subjective pain sensation. It is actually obvious and easy to follow.
Use it, but when it hurts, stop.
Yeah, but given the huge variations in pain tolerance, some will reinjure it while others will never use it again.
Yeah, but given the huge variations in pain tolerance, some will reinjure it while others will never use it again.