Harbaugh's pattern is to coach 4 years or so, and then leave for something else. He either gets bored with what he is doing or is asked to leave after a few years. I suspect he stays for at least another year or two, but...
Perhaps someone who follows Michigan football more closely can answer these questions. How many starters is Harbaugh losing from this year's team? Will next year's Michigan team be dominated by young players, rather than a large core of upperclassmen? Harbaugh has recruited some good, young players, but will they be ready to achieve another 10 or 11 win season as early as next year?
If Harbaugh sees a couple seasons of rebuilding ahead of him, he just might take another shot at the NFL.