In my estimation, 28-30 years - long enough for his son to guide us to a national championship and then said son will get into coaching, work his way up to take over for Papa after around 6-8 years of experience.
He leaves in 4 years to replace a retiring Urban Meyer at his dream job at Ohio State. Nebraska then replaces him with a roughly 70 year old and recently fired Herm Edwards coming off a 4-44 stint at Arizona State.