Seriously, as long as a graphite shaft is not cracked and there is no sign that the layers of the graphite material are starting to delaminate (peel away), the shafts will last for as long as you have the clubs.
In steel shafts, as long as the shafts are not kinked, or badly pitted or rusted, they, too, will last forever and will never wear out.
It is a myth to think that any shaft will "wear out" or suffer from "fatigue" to the point that it no longer performs the same - so long as the shaft is not damaged or bent.

