How old is your DC? Timing depends on whether you are applying to Early Action (EA, not binding), Early decision (ED, binding), rolling admission (can send in anytime after junior year in most cases); or regular decision (RD). A lot of EA and ED are due by Nov 1 or December 1 of senior year so you want to take the exam starting in Junior year (at least once) and try again before app is due beginning of senior year. Every family uses slightly different path so there is not one "right answer".