Is there definitive information about how long dropped items stay around for a way of permanent storage?
Not a solution to your original question, but a solution for your permanent storage problem: About two weeks ago, CD Projekt Red announced patch 1.07 which will add a permanent storage.
A player stash for storing items, available in various locations throughout the game. Stash locations are marked on the player's map.
See this forum post and the official change log.
CD Projekt Red didn't give any details about the release date of the patch. They only stated that the PC version is ready and the console versions are waiting for certification.