Im taking prenatal and clomid at the same time 2nd round..can somebody tell me wen 2 take robitussin n hw much?

im on my 2nd round of clomid day 5-9 50mg....iv just started taking one a day prenatal pills+DHA the same day i started my clomid..im on day 3 of my clomid pills when should i start taking the robitussin and how much a day should i take n for how long.i got the one with only guaifenesin 100mg as the active ingrediant