The Shulchan Aruch (123:2) rules that after a person completes the Amida, steps back three paces, and recites "Oseh Shalom," he should not return to his place immediately. Rather, he must remain in the spot where he recited "Oseh Shalom," with his feet together, until the Chazan begins the repetition, or, preferably, until the Chazan reaches Kedusha. At that point he should then step forward three paces beginning with his right foot to the spot where he had stood for the Amida. Many people return to their original place immediately after stepping back and reciting "Oseh Shalom"; this is incorrect. One should instead remain standing in place at least until the Chazan begins the repetition, and preferably until Kedusha. It should also be noted that one should not pass in front of a person who had completed the Amida and stepped back until he has returned to his place.
A person praying privately, such as at home, must likewise remain in his place after stepping back at the conclusion of the Amida. He should remain there for at least the amount of time required to walk four Amot (6-8 feet), but preferably he should wait the amount of time it would normally take the Chazan to reach Kedusha after beginning the repetition, or approximately 15-20 seconds. Similarly, if a person prays with a Minyan but finishes the Amida only after the congregation has recited Kedusha, he should remain in his place after stepping back for at least the amount of time required to walk four Amot, and preferably the amount of time it would take the Chazan to reach Kedusha.
(These Halachot are based on Halacha Berura, Siman 123, pp. 235-6.)
Summary: After a person completes the Amida, steps back three paces and recites "Oseh Shalom," he should remain there with his feet together until Kedusha, and only then return to his place. If one prays privately, or if one completes the Amida after the congregation recited Kedusha, he should remain in his place for the amount of time it would take the Chazan to reach Kedusha after beginning the repetition, or approximately 15-20 seconds.
