Well, you would need a quite large inventory of lithium in the reactor to capture a large fraction of the neutrons, but it would only be consumed at a slow rate. Even assuming only 20% of the fusion power comes out as net electricity output (the rest being either lost as waste heat or needed to keep the fusion going), a 1 GW D-T fusion power plant would consume only about 275 kg of tritium per year, which would correspond to a lithium consumption of about 600 kg per year, depending on the specific mix of lithium isotopes.