Under the assumption of time-dependent net core breeding gain of fusion-fission symbiotic systems, analytical forms for the stockpile fuel trajectories have been derived, both for continuous and discontinuous operation. Also, usable regions for characteristic system parameters such as doubling time, minimum inventory, minimum inventory time, and minimum initial inventory have been defined. It is shown that fuel trajectories defined previously are underestimating the importance of the time dependence of the net core breeding gain.