I actually looked at this for a blog post that I wrote recently. I wanted to see how long it took before a coach won a national championship, and I arbitrarily chose the past 20 years as a sample size.
Here is what I wrote:
Let us look at a list of all Coaches in the past 20 years who have won a National Championship.
Bobby Bowden, Florida State (1993/1997/1999)
Tom Osborne, Nebraska (1994/1995)
Steve Spurrier, Florida (1996)
Lloyd Carr, Michigan (1997)
Phillip Fulmer, Tennessee (1998)
Bob Stoops, Oklahoma (2000)
Larry Coker, Miami (2001)
Jim Tressel, Ohio State (2002)
Nick Saban, <--the devil
Pete Carroll, USC (2004)
Mack Brown, Texas (2005)
Urban Meyer, Florida (2006/2008)
Les Miles, LSU (2007)
Gene Chizik, Auburn (2010)
The average number of years that any of these men coached at a school before they won their first national championship was 5.9. If you remove the two late bloomers from the list, Bowden and Osborne, the average drops to 2.6 years.
What does this illustrate?
In 20 years, 86% of coaches who won a national championship averaged 2.6 years from being hired to winning it all. The two others (14%) averaged 19 years from their start to the national championship.