ggL_03

I was given distilled water, starch solution, sodium thiosulphate, potassium iodide solution and potassium persulfate solution to perform the clock reaction as attached. I want the mixed solution turning into blue black solution after 14 seconds. Therefore, exactly how much concentration of sodium thiosulphate to complete the reaction in 14 seconds???? I don't know where to start and what formula to use.