You can't really diagnose by levels, though, unless you knew what that person's previous levels were. Setting an average across a population is not really realistic - you can't say Shaq should work to the same levels as, say, Emo Phillips.
TRT is normally used due to aging, though, so you are unlikely to have your testosterone levels spontaneously recover as you get older. You do tend to need to be on it for life, in the same way that women stay on HRT.
However, if you did need to get off, bodybuilders have "post cycle therapies" to kick start production so it seems to be possible.