I have pondered how many poor seasons it takes before a program simply fades from view, how much turnover with the HC, etc. Obviously, it depends, Cincinnati could do it in 2 years probably. Michigan? They have not imploded like Tennessee of late, I'd opine Tenn is on the precipice.
Another coach with a rebuilding job and no guarantees of doing much better in 2-3 years.
Of course, a great or near great coach can turn it around quickly.
I think, like most of anything, it comes down to recruiting.
Michigan still gets good recruits, even through the Rodriguez/Hoke coaching years. Notre Dame, as they went through the Davie/Willingham/Weis coaching run, still got good recruits. I don't know as much about Alabama, but I'll bet that between Stallings and Saban they didn't fall off the map completely regarding recruiting.
That, to me, is the key. If you're still recruiting well through non-performing coaches, you're still a helmet team. When the recruiting fades, is when you're in danger of falling off.
That's also the difference with coaches that are overperforming. If you have a coach that is really good at spotting, coaching up, and developing talent, we'll say a Ferentz or a Dantonio, you can look really good. But if the recruiting actually doesn't uptick, as soon as that coach is gone you see a falloff.
Football success at the collegiate level* is a function of talent+coaching. The "haves" are the haves because they have the talent, regardless of whether they have good coaches. When they get a coach that matches that talent, they're perennial NY6/CFP contenders. The have-nots, however, may have great outlier years where they might see the NY6 or CFP based on coaching rather than talent [and luck], but when they get there and face a team that has both talent & coaching, they get exposed.
* Truth is it's talent+coaching at all levels. But in college, talent is based on recruiting, which is a naturally unequal playing field. In most pro sports, there are drafts and salary caps, which brings all teams much closer to parity in talent levels unless they've got horrible management.
