According to the latest release, the leaderboards should work like this: Leaderboard times now remain valid while you are riding in Zwift and for one hour after. If you log off and return within an hour, you will regain your jersey if your time remains the fastest.
However, I noticed that my Orange jersey time expired during a ride (probably at about an hour after it was set, but I was not keeping that close attention). Additionally, times set less than an hour ago were gone as well (they would have been on the leaderboard after my fastest expired). Also, after that point that the original expired, I set new times that should have been on the leaderboard, but they did not show up either. It was basically like I was no longer eligible for the orange jersey, regardless of when the time was set during the ride.