As long as he wants, although the FAA is supposed to put out or drop the case by six months. If longer than six months, you'll have the stale complaint rule to use in addition to your defense.

Although, you might want to inquire of a safety program manager (white hat fed), FAAST mangers now I think, to see if they can check the case status (PTRS) without kicking the 'sleeping dog'.

Good luck!