alt Hacker NewsThe RFC does say "It is RECOMMENDED that all senders and recipients support, at a minimum, URIs with lengths of 8000 octets in protocol elements."
One can infer from the RFC that you can reasonably expect many implementations to fail beyond 8000 characters, and that there are no guarantees up to that either.
True, the RFC doesn't specify a limit, but it does clearly indicate that it's not unbounded, nor should you expect it to be.
This RFC (10008) does not require caching to be implemented at all, so it would make no sense to make a recommendation here for what is a reasonable limit to expect caching to work.