Looking at elite teams since the NU BobFather era began. AP National Championships by the top all time winning teams in history/
Alabama went from 66 to 79 to 91 to 2009 - roughly every 20 years between MNC
Texas went from 70 to 2005 - 35 years
OU from 74, 75 to 85 - 2001 - roughly every 20 years
Notre Dame 66, 73, 77, 88
USC 62, 67, 72, 2003, 04
NU 70, 71, 94, 95,
Michigan 97
Ohio State 68, 2002
I think you see my point? When you look at actually winning national championships (playing for a national championship is usually undefinable in a non-play off system, for example, did Alabama "play" us for a nationalship in the orange bowl in 1971 or would OU have been #1 if we lost to Alabama?) see here for examples of how many teams were considered national champions by various entities since 1960.
http://en.wikipedia...._Division_I_FBS
So, to say it could take 15 years under Bo to create a perfect storm of talent, attitude, luck, schedule and coaching to win a national championship is consistant with what history teaches us about college football "dynasties"