Well that's a complicated question. Circulation is the biggest factor, but also environmental conditions must be taken into account : burial, salt air on coasts, poor storage (PVC), etc etc. But stored adequately, a coin's lustre will last indefinitely, though it may tone and/or fade. But it is still there, technically. As Coinery says, a shimmering but toned coin (copper/bronze) can be fully lustrous! Just make sure you store properly - PVC-free plastic flips, cabinets, trays are the best. Keep free from environmental pollutants, but don't get too hung up about lustre fading or toning. It's not the same as circulation wear, and only the money of determined but ignorant buyers will say otherwise.