Does the "Club of Helmets" have a fixed number as a limit?
I agree with
@FearlessF :
don't think so
I also agree with
@847badgerfan :
Pretty hard to get kicked out of that club.
Some of the Michigan fans have argued that the "Helmets" were permanently set as of the 1970's and can never change. I strongly disagree with this. Minnesota was a "helmet" in the early days of CFB up until WWII and they clearly are NOT now. I think that the same thing can and will happen to teams that we think of as helmets today.
That said, Badge has a good point. Once you ARE a helmet, you don't fall out of that club for having a bad year or two or even necessarily for having a bad decade or two. However, if you fail to "look like" a helmet for long enough, eventually you will no longer be a helmet. Exactly how bad you have to be and for how long for this to happen are not explicitly stated but I strongly believe that the possibility exists.
Another way to put it is that maintaining helmet status is considerably less demanding than attaining helmet status. Consider, for example, if Wisconsin and Michigan met in this year's CFP Championship game:
IMHO, the outcome of the game would be irrelevant to Michigan's "helmet status". They already are a helmet and advancing to the CG is sufficient to remind everyone that they ARE a helmet. Losing that game wouldn't hurt them at all and winning wouldn't really help because simply getting there would already have done more than enough to maintain Michigan's "helmet status".
Conversely, the outcome of the game would be HIGHLY relevant to Wisconsin's "helmet status". They aren't a helmet and winning NC's is a REALLY good way to become a helmet. Getting to the CG is great but plenty of non-helmets have been to CG's. Oregon made it in 2014. Clemson wasn't a helmet yet when they first got there. Auburn and VaTech both got to the BCSCG.