Determining the Reactant Concentration at a Given Time?

At 25�C, hydrogen iodide breaks down very slowly to hydrogen and iodine: rate=k[HI]2. The rate constant at 25�C is 2.4 10-21 L/mol�s. If 0.0155 mol of HI(g) is placed in a 1.0 L container, how long will it take for the concentration of HI to reach 0.0085?