This is an old one which obviously was missed at the time. Sorry!
I have seen this many times so I thought I'd dredge it up briefly.
I'm not swure if you can come up with a simple policy but written guidelines would be nice. What I have seen is typically some guidelines but the borderline cases are simploy a decision made by the person responsible for issuing and/or accepting or rejecting corrective action responses. This is because many times the situation is so unique.
I have seen many companies fall into the trap of: <ol><li>requiring corrective actions for practically anything and everything and ending up with a pile which never are addressed, and<li>I have also seen it where those generated were appropriate but getting people to follow through is next to impossible with the same result - a pile which will never see the light of day.</ol>
Registration to ISO 9K or an equivalent such as TS 16949 should help ensure the pile doesn't exist (or doesn't grow) by forcing management to act.
I can't tell you where to set the 'limit' or how to determine where to set it without more details, but if anyone wants to post some specifics we can discuss