Is that useful to clean them or you mean to replace because of wear and tear ?   I also found fans to be the most likely to slow or stop mining, in one case it was the actual fan controller chip that went wrong without any good reason for it.

I have a XFX card from 2008 thats never stopped, it was a blower style though which apparently do last longer.   Not anymore but at one time XFX offered life time warranty to original owners, seems they are still higher quality like that