This question is about a nearly decentralized P2P system with a centralized certificate authority where users can authenticate and receive a certificate on their public key and then can authenticate with peers using that certificate - independently from the certificate authority. In short, this is about morally equal to a "keep me logged in" checkbox that uses a cookie.
If the certificate gets stolen the user can be impersionated until the certificate expires, especially in a P2P system where certificates are nearly impossible to retract. This is an argument to make the expiry quite short, say an hour.
On the other hand we have the argument of dependency and performance. If the expiry is very long it reduces stress on the certificate authority, and it also reduces the impact when it is unavailable.
What is a reasonable expiry for such "logged in" certificates?