"When do you know enough is enough when dealing with a subordinate that doesn't seem to be helping?"

This is another good management question. The best way to answer this question is with a procedure in mind. Basically after you tried everything, then enough is enough. But what exactly is everything. You can't use everything because that shows you really don't know. Instead of the word everything, list out all the steps so the interviewer believes you know what to do in this situation.

"I know enough is enough when I've exhausted all measures to help this employee grow. I would do my part by providing encouragement, giving extra instructions, mentoring, and giving different types of work. If I exhaust all of these steps, then I know I can't do anything further and that is when enough is enough."