Gemara

(Mishnah): If one said 'this is your Get on condition that you nurse my son', the period of nursing is two years;

R. Yehudah says, it is 18 months.

Rava holds that this is how long she must nurse when he did not specify.

Rav Ashi holds that one day suffices. He holds that the Mishnah teaches that the day must be within the first two years (or 18 months) of the son's life.

Kesuvos 60a (Beraisa - R. Eliezer): A child may nurse for 24 months. After this, it is like nursing from a disgusting thing;

R. Yehoshua says, he may nurse for four or five years. If he stopped nursing after 24 months and resumes, it is like nursing from a disgusting thing.

(Beraisa - R. Meir): If a nursing woman was widowed, she may not accept Kidushin or have Nisu'in until 24 months (lest she become pregnant and wean the baby prematurely);

R. Yehudah says, she must wait until 18 months.

R. Nasan bar Yosef says, they argue as Beis Shamai and Beis Hillel do. Beis Shamai say that the Shi'ur is 24 months. Beis Hillel say that it is 18 months.

R. Shimon ben Gamliel says, the opinions that say 24 and 18 permit to marry after 21 and 15 months, respectively, because milk is not affected until three months of pregnancy.

Rav Yosef: Rav and Shmuel both require waiting 24 months, excluding the day of birth and the day of Kidushin.

Nidah 9a (Beraisa - R. Meir): If a woman was nursing and her baby died within 24 months, she is (prone to see blood) like regular women.

Erchin 31b (R. Aba bar Mamal): If Reuven bought a house in a walled city on Adar Rishon 15 in a leap year, the year of redemption is (13 months) until Adar 15 next year. If a lamb was born on the 15th of Adar Rishon, it completes its year on Adar 15 of the coming year.

R. Aba needed to teach both cases. One might have thought that the extra month is included only regarding Bayis Ir Chomah, for there it says "Temimah". R. Aba teaches that this also applies to a lamb; we learn from a Gezeirah Shavah "Shanah-Shanah."

(Mishnah): It says "Temimah"...Rebbi says this includes the extra 11 days in a solar year.

(Beraisa - Rebbi): "Shanah Semimah" - we count 365 days, a solar year;

Chachamim say, we count 12 months. If it is a leap year, we count also the extra month.

Rishonim

The Rif and Rosh (37a and 6:10) bring our Mishnah.

Rif and Rosh (Yevamos 14b and 4:16): The Halachah is, one may not be Mekadesh a nursing or pregnant until 24 months, excluding the day of birth and the day of Kidushin.

Rambam (Hilchos Gerushin 11:25,26): One may not marry a nursing woman. The period of nursing is 24 months, excluding the day of birth and the day of Kidushin.

Poskim

Shulchan Aruch (EH 13:11): Chachamim decreed not to be Mekadesh Me'uberes or Meinekes Chavero until the baby is 24 months old.

Rema: This is according to the months, be they deficient or full (29 or 30 days).

Shulchan Aruch (ibid.): The 24 months exclude the day of birth and the day of Kidushin. The Ibur month (Adar Sheni) counts towards the 24 months.

Rema: Some say that l'Chatchilah, one should be stringent not to count the Ibur month.

Beis Yosef (DH u'Mah she'Chosav Od, citing the Mordechai): Maharam requires 24 full months. If it is a leap year, the baby gets an extra month. Some Gedolim require 24 lunar months alternating between short and full, i.e. 708 days. Those who learn two years of nursing from "Lo Sevashel Gedi b'Chalev Imo" (the Gematriya of "Sevashel" is 732) require 24 (alternating) months and twice the extra 11 and (almost) a quarter days in a solar year. (In all, it is about 730 and a half days.) Those who learn from Shmuel ha'Navi (Chanah nursed him for two years, then brought him to Eli) say that it is two years, be they leap years or full years. However, the conclusion in Erchin connotes that it is only 24 lunar months. (We give a full year for Bayis Ir Chomah only because it says "Semimah" (and regarding a lamb, due to a Gezeirah Shavah). Here, there is no Drashah to teach about full years.)

Maharam (brought in Chidushei Anshei Shem on Mordechai, Sof Kidushin [2]): In the Mishnah, R. Yehudah requires 18 months. Surely, the first Tana mentioned two years because he holds that it does not depend (only) on months! In the Seifa, he discusses 12 months! Rather, we are stringent for whichever is longer, two years (if one of them is a leap year), or (if not,) 24 months (of 30 days). We cannot learn from Erchin, which expounds verses. Chachamim fixed the period of nursing!

R. Yitzchak (in Chidushei Anshei Shem, ibid.): In Kesuvos and throughout Shas, only the Shi'ur of 24 months is mentioned. We need not be concerned for two years.

Ben Lev (2:21 DH Od): Rashi (75b DH Rebbi) says that the argument here is the same as in Kesuvos. I.e., Chachamim require two full years. There, it says 24 months lest one mistakenly think that for any baby, its second Rosh Hashanah is called two years. This would endanger babies born at the end of the year!

Note: Rav and Shmuel were meticulous to say '24 months, excluding the day of birth and Kidushin.' Why didn't they mention the Ibur month?! Maharam himself mentioned that the Torah, Chachamim and people speak differently. Perhaps Chachamim knew that 24 months is the proper time to nurse, but people think that it is two years, and stipulations follow the intent of people. Therefore, only in Gitin the Shi'ur is two years!

Terumas ha'Deshen (216, cited in Darchei Moshe 7): L'Chatchilah, one should be concerned for the Ibur month. If one was Mekadesh her in the extra month, we do not force him to divorce her.

Beis Yosef (DH v'Hagahos and DH ul'Inyan): Hagahos Maimoniyos (Hilchos Gerushin 11:40) says that since Chachamim fixed the Shi'ur in months, not years, this shows that the Ibur month counts towards the 24 months. Chachamim knew that 24 months suffices for a baby, even if the Ibur month is included. We follow his sound reasoning.

Igros Moshe (EH 2:9 DH Aval): Chachamim voted to forbid Meinekes Chavero, without fixing the period, for it is how long women nurse in every generation. R. Yehudah forbids for 18 months, for most women nurse this long. R. Meir is concerned for the minority, who nurse 24 months. Nowadays, nature has changed. In our lands even a minority does not nurse past 18 months. We are not concerned if a tiny minority, or women elsewhere nurse for 24 months. Even though most women here do not nurse at all, and those who nurse do so only a few months, we do not permit before 18 months. We are concerned for a minority who nurse this long elsewhere.

Rebuttal (Tzitzis Eliezer 19:38): Chachmas Shlomo (EH 13:11) said that nature has changed and nowadays women do not nurse longer than 18 months, so this suffices. However, he wavered, and did not permit against all the Acharonim. The Chasam Sofer (EH 1:30 DH u'Lehotzi) denounced those who permit because nature has changed. One can join this with other reasons to permit Kidushin, e.g. she weaned the baby before divorce (some permit any divorcee right away) and intends to have Nisu'in after 21 months (R. Shimon ben Gamliel's Shi'ur).

Shevet ha'Levi (8:269 DH Hinei): One may rely on the Chachmas Shlomo to permit one who weaned her baby after two solar years, without waiting for the Ibur month.

Igros Moshe (4:50 DH Ach): If a divorcee weaned her baby after 12 months she is permitted, since almost no women in our lands nurse longer than this.