When there is no solid present.
If one has a solution of lead nitrate, and one added sodium chloride, at some point the concentration of chloride will be such that the ion product(Qsp): [Pb++][C? -]2 will exceed Ksp whereupon precipitation will occur.
This means that one needs to do a trial an error calculation when presented with the ionic concentrations, asking whether or not the ion product exceeds Ksp? If it does, then precipitation occurs, lowering all ionic concentrations until…