FeCl3 reacts with the iron in the barrels and the rust by moving electrons.The Fe is in the +3 state and will snare one electron per atom and become Fe in the +2 state. So, how long will this take? It all depends on how much etching is going on. In your case, not very many barrels, as the iron and rust have a lot of electrons to furnish and your gallon has a limited number of electrons to take .It is possible to regenerate the FeCl3 from the FeCl2 that was generated,but that is dangerous chemistry.So, as the process slows to nothing,time to replace.
