Nedarim 44: Hefker Remorse

On the rules of "hefker," declaring something ownerless. And how it removes this issue of being unable to provide benefit (or not). How long is a declaration of hefker into effect? And when can that declaration be retracted? That is, if nobody else has come along to claim the "ownerless" property. Also, timing that is designed to protect against swindlers. Plus, when the Torah requirement for tithing and the rabbinic standing overlap in ways that leave produce untithed when it should be tithed.