Gemara

12b - Mishnah: The year of Ma'aser for olives and Tevu'ah (grain; Rashi - and grapes) is determined by when it grew a third.

Question: What is the source to follow one-third growth for olives and Tevu'ah?

Answer (Rav Asi) Question: "Mi'Ketz Sheva Shanim b'Mo'ed Shenas ha'Shemitah b'Chag ha'Sukos" - this (Hakhel) is during Sukos after the end of seven years, in the eighth year. Why is it called Shemitah?

Answer: This teaches that any Tevu'ah that was a third grown before Rosh Hashanah (at the end of Shemitah) has Kedushas Shemitah.

Question (R. Zeira): Perhaps no matter how much grew before Rosh Hashanah, the Torah forbids to plow or harvest it before Sukos!

Answer #1 (Rav Asi) Question: "V'Chag ha'Asif b'Tzeis ha'Shanah" - why is it called 'Asif'?

Suggestion: Perhaps Sukos is called Chag ha'Asif because it is at the time of gathering (produce that was drying in the fields)!

Rejection: The verse already says "b'Ospecha"!

Answer: Rather, it refers to harvest (from Sukos and onwards, what is harvested is considered produce of the coming year. Chachamim know that any Tevu'ah harvested on Sukos grew a third before Rosh Hashanah.

Rejection (R. Chanina): We learned that "B'Ospecha mi'Gornecha umi'Yikvecha" discusses (using for Sechach) the waste products of the threshing floor and winepress (Ospecha does not refer to harvesting)!

Answer #2 (Beraisa - R. Yonason ben Yosef): "V'Asas Es ha'Tvu'ah il'Shlosh ha'Shanim" - we read this 'Shelish'.

Question: The verse is not extra (it explains how we will have what to eat in Shemitah and the following years)!

Answer: We already know this from "...V'Ochaltem Min ha'Tvu'ah Yashan Ad ha'Shanah ha'Teshi'is".

Shevi'is (4:9): In Shemitah, if olives have grown enough to produce a Revi'is (of a Log of oil) per Se'ah of olives, one may bash them (to sweeten them) and eat them in the field;

If they produce a half-Log per Se'ah, one may crush them and anoint in the field;

If they reached a third, one may crush them in the field and take the oil into his house. Corresponding to this they are obligated in Ma'aser in other years. For all other fruits, the time for Ma'aser and Shemitah are the same.

Toras Kohanim (Behar 1:11,12): Olives that produce three Lugim for a Se'ah (when fully grown):

If they (have grown enough to) produce a Revi'is, one may bash them and eat them in the field. If they produce a half-Log, one may crush them and anoint in the field. One may crush them in the field and bring them into his house if they produce a Log (the text of the Ra'avad and R. Shimshon says 'a third', but they explain that this refers to a Log). Corresponding to this they are obligated in Ma'aser in other years.

Rishonim

Rambam (Hilchos Ma'aser 2:5): The time for Ma'aser for olives is when they produce a ninth as much oil as fully grown olives. This is one-third growth.

Rebuttal (Ra'avad): The Rambam was misled by his text in the Yerushalmi Shevi'is (4:7). Our text reads 'what is a third? A Log (of oil per Se'ah of olives). Our Mishnah is where normally a Se'ah produces three Lugim, i.e. poor land. The Rambam's text read 'what is a third? A third of a Log...'

Defense (Ri Korkus): According to the Rambam, the Tana taught that it depends on a third, i.e. a third of a Log which is a ninth of the full yield in an average place. But according to the Ra'avad, the Mishnah (which merely says 'a third') does not teach the Halachah, for there is no hint that it discusses poor places which produce only three Lugim. Also, the previous Shi'urim in the Mishnah refers to a quarter or a half of a Log. This suffices to favor the Rambam (in spite of difficulties brought below).

Question (against Ra'avad - Kesef Mishneh): The Rambam's text is better. According to the Ra'avad's text, the Yerushalmi should not have commented, and we would know the Halachah from the Mishnah!

Answer (Mishneh l'Melech): The Yerushalmi teaches that a Log is the Shi'ur in every place!

Question (Ri Korkus): According to the Rambam, when it produces a half Log one may anoint only in the field, because it is not called Tevu'ah. But when it produces a third, it is considered like full development, and this is also the Shi'ur for Ma'aser! Perhaps half a Log discusses where olives normally produce more than nine times as much, and the Yerushalmi only discusses the last clause. This is difficult. Perhaps the middle clause discusses a half-Log of oil from what is now a Se'ah of olives; if the olives would be fully grown, they would be (e.g.) two Sa'im, so the oil obtained is less than a ninth of fully grown olives.

Mishneh l'Melech: Indeed, the first clauses of the Mishnah teach that whenever olives produce a Revi'is (or half Log), one may eat (or anoint) in the field, even if this is much less than a ninth of the yield of full growth.

Ri Korkus: Olives were taught together with Tevu'ah, and regarding Tevu'ah surely we discuss third growth. This is why the Rambam said 'this is their third', i.e. it is a ninth of the yield, but the verse calls this 'Shelish'. It is like a third growth of Tevu'ah regarding Ma'aser. The Toras Kohanim supports the Ra'avad, and R. Shimshon explains like him.

Rashi (12b DH veha'Zeisim): The year for olives depends on when they ripened a third.

Rambam (ibid.): The Shi'ur for Tevu'ah is when it one-third growth.

Rambam (Hilchos Terumos 2:10): If Tevu'ah or olives did not reach one-third growth it is exempt from Terumos and Ma'aseros. One can test this by planting it. If it sprouts, it is a third grown. If one tithes before this, it does not take effect.

R. Shimshon (Shevi'is 4:9): It is normal to eat olives that produce a Revi'is, but it is not normal to anoint with them. 'If they reached a third' means, they are a third of their full growth. Alternatively, they produce a third as much oil as fully grown olives.

Rosh (11): His first Perush is primary, for the time for Ma'aser is one-third growth.

Poskim

Shulchan Aruch (YD 331:15): If Tevu'ah or olives did not reach one-third growth it is exempt from Terumos and Ma'aseros. One can test this by planting it. If it sprouts, it is a third grown. If one tithes before this, it does not take effect.

Source: Sanhedrin 69a (as explained by Rashi DH she'Lo - Gra 33) and Yerushalmi, brought in Radvaz (Terumos 2:10).