19:11 If someone forgot to say ya’aleh v’yavo at shacharis on Rosh Chodesh or chol hamoed, he must repeat Shemoneh Esrei, even if he already said musaf including ya’aleh v’yavo. (There are a variety of opinions on this matter. One may be able to rely on his ya’aleh v’yavo from musaf if he davened with a minyan but perhaps not if he prayed alone - see Mishnah Brurah 422:4 for more on this.) If the time in which one can daven shacharis has already passed, he adds a compensatory prayer at mincha. 19:12 Whenever one is obligated to repeat Shemoneh Esrei, he must wait the amount of time it would take to walk four cubits (six feet) before beginning again.