When is the Torah scroll to be returned to the ark on Rosh Chodesh?
Chassidim and Sephardim: The custom of Chassidim and Sephardim is to return the Sefer Torah to the Aron after Uva Letziyon and Kaddish Tiskabel. This applies likewise on Rosh Chodesh. Thus, on Rosh Chodesh the Sefer Torah is returned to the Ark only after Uva Letziyon, prior to the recital of the half Kaddish before Musaf. [This is also the Chabad custom, however some Shul’s of Anash return the Sefer Torah to the ark immediately after Gelila is completed.]
Ashkenazim: The custom of Ashkenazim is to return it before Ashrei. This applies likewise on Rosh Chodesh.
 Siddur Admur; Custom mentioned in Admur 25:39; 1st custom mentioned in Rama 25:13; Michaber 423:3; Second opinion in Tur 149; Siddur Arizal; Mishnas Chassidim; See Shaar Hakolel 22 [p. 39]
The reason: As the Divine light drawn down through prayer concludes after the entrance of the Sefer to the Heichal and we hence desire to delay this as much as possible. [Shaar Hakolel ibid; See however Kaf Hachaim 135:2 in name of Shaar Hakavanos that states it is for this reason that we return the Sefer Torah immediately after the reading]
Other opinions: Some are accustomed to return the Torah prior to Kaddish Tiskabel. [Piskeiy Teshuvos ibid in name of Kol Bo and Abudarham]
 Michaber 423:3; Second opinion in Tur 150; Siddur Admur; Custom mentioned in Admur 25:39; 1st custom mentioned in Rama 25:13; See Shaar Hakolel 22 [p. 39]
 Siddur Admur regarding Rosh Chodesh “Afterwards one says Ashrei and Uva Letziyon and enters the Sefer Torah to the Heichal”; Hiskashrus 457 p. 18; Siddur of Rav Raskin p. 482; Likkut Dinei Rosh Chodesh 7:15
 Background of Chabad custom: In the Siddur of Admur he writes explicitly to return the Sefer Torah on Rosh Chodesh after Uva Letziyon, and so was the Chabad custom in all places until the year 1988. From the year 1988 it became accustomed in 770 to return the Sefer Torah to the Ark immediately after Gelila and not to wait for Uva Letziyon to be recited. This is unlike the Chabad custom followed throughout the year on Mondays and Thursday’s, which follows the ruling of the Michaber. Some suggest the reason for this custom is because it is not our custom to end Uva Letziyon aloud in order to avoid the necessity to say Kaddish immediately afterwards [see next Halacha] and hence in order to negate a mistake the custom became to immediately return the Sefer Torah to the Heichal, hence signifying the conclusion of the Chazan’s part of Davening. [See Hiskashrus 466 p. 15]