When does the amidah end?

Hi,

Yesterday, we noted that one who omits "Yaaleh v'Yavo" in the Rosh Chodesh amidah (at shacharit or minchah), and realizes his error before stepping out of the amidah, returns to "Retzeh" and continues from there.

The Mishneh Berurah clarifies that even if one has not physically stepped out of the amidah, he is deemed to have concluded the amidah once he has decided not to say anything further, even if he has not actually stepped away.

(Mishneh Berurah 422:9)

Have a great day,
Mordechai