Formation Constant — Definition & Role in Equilibrium

(2016 U.S. National Chemistry Olympiad Local Exam #36) The tetramminecopper(II) ion, $\text{Cu(NH}_{3}\text{)}_{4}^{2+},$ has a formation constant $K_f = 1.1\times 10^{13}.$ What is the minimum concentration of free ammonia in solution required to ensure that at least $99.9\%$ of the dissolved copper(II) ion is found in the form of its ammonia complex?