Completely agree with cecropia_moth. I think most silver was pulled out of circulation within the first decade of the switch to clad. Based on accounts I have seen, a lot of people were very suspicious of the move away from silver coinage and kept any silver they had, not to the point of CRHing like we do but just keeping any in their change jars and that they would get in change. A lot of it was thrown back in after silver crashed in the 1980s, and other amounts have trickled back in as collections have been cashed back in unknowingly after a death or theft, or even knowingly by people in desperate financial situations who needed fast cash.
The amount of silver we find that has been circulating unnoticed since the change to clad is probably very low. I'd say less than 10% for halves, dimes maybe 5%, and quarters even less (1-2% at most). Remember that CRH has been around a long time (I've seen references to CRHing in the 1970s on this forum), so chances are the silver coin had been pulled at some point.