The Gemara (Bava Metsia 31a) writes that based on the double language of the pasuk, there is an obligation to continue to give rebuke even 100 times. The Rambam (Dei’ot 6:7) rules like the opinion that one is obligated to continue to rebuke until the sinner strikes the one giving reproof and declares that he is not going to listen. Bei’ur Halacha 608:2 s.v. Ad quotes the Sefer HaChinuch, who clarifies that the obligation applies only until the sinner is ready to strike the one giving reproof; there is no obligation to wait until he actually strikes.

Bei’ur Halacha 608 s.v. Aval writes that the mitzvah of Tochacha doesn’t apply to a person who has totally rejected the yoke of Torah such as someone who violates Shabbat in public. The reason for this is that he is not included in the Torah’s description of “one’s fellow.” Aruch HaShulchan 608:7, Tzitz Eliezer 17:36, and Shevet HaLevi 1:205:608 agree.

The Gemara (Arachin 16b) learns from the end of the pasuk that it is forbidden to give rebuke if it will cause one to embarrass the one receiving rebuke. Thus, the Rambam (Dei’ot 6:7) and Sefer HaChinuch (239) rule that one should begin to give rebuke in a gentle tone, using soft words, and in private. Kitzur S”A 29:15 concurs. Rabbi Aryeh Lebowitz (The Mitzvah to Rebuke) points out that before giving anyone rebuke, one must be absolutely sure that the person actually did a sin and wasn’t simply following an acceptable posek or minhag. Thus, Rav Moshe Feinstein (Igrot Moshe EH 4:63) writes that just like any other halachic inquiry, questions regarding rebuke must be asked to a posek so as not to violate the serious prohibitions including ona’at devarim.