Peah Chapter 8, Mishnah 1. This mishnah addresses the question of the point from which the gifts to the poor may be taken even by those who are not poor, by the general public. The Rambam explains that the obligation of gifts to the poor applies as long as the poor are still pursuing them; once the poor are no longer interested in them, the Rambam derives from the verse that they become permitted to everyone.
Leket (and according to one view, shichechah and peah as well):
The mishnah opens: "Me'eimasai kol adam mutar b'leket" - from what stage in the agricultural process may any person take leket. There is a question whether the law applies to shichechah and peah as well: the version of our mishnah that the Gemara in Maseches Taanis had before it includes the words "shichechah and peah," while others do not have this version.
And what is that stage? "Mishe'yelchu ha'nemushos" - the nemushos are those who walk slowly, the elderly and the weak. Once they have passed through the field and left it, the matter becomes permitted, since they are the last stragglers, the final group among the gatherers.
Peret and olelos:
Peret is like leket except that it applies to grapes - the grapes that fall during the harvest; and olelos are the individual grapes that grow not in a cluster. From when do these become permitted? "Mishe'yelchu ha'aniyim ba'kerem v'yavo'u" - once the poor have passed through the vineyard twice, whatever remains afterward is permitted, since at this stage the interest of the poor in it has already ceased.
The Yerushalmi explains why for peret and olelos we do not use the measure of the departure of the nemushos: the nemushos, the elderly and the weak, were more interested in the grapes of peret and olelos than in the grain, and therefore they hurried to reach them before the grain. For this reason no separate time of "until the nemushos have gone" is set for them; rather, the time is when the poor have passed through the field a second time.
Olives:
And with olives - "Mishe'teired revi'ah sheniyah". Olives are permitted after the second revi'ah (rainfall) of the autumn has come down. The Gemara in Taanis explains that there are three fixed times for the rains to fall, and the Tannaim disagree about them:
Rabbi Meir: the third of Marcheshvan - the first revi'ah; the seventh of Marcheshvan - the middle revi'ah; the twenty third of Marcheshvan - the last revi'ah.
Rabbi Yehudah: the seventh, the seventeenth and the twenty third of Marcheshvan.
Rabbi Yose: the seventeenth and the twenty third of Marcheshvan, and Rosh Chodesh Kislev.
According to all the views the reference is to the middle revi'ah of the three autumn rains, and once that rain has fallen, the olives are permitted.
The view of Rabbi Yehudah:
Rabbi Yehudah disagrees with setting the time at the second revi'ah, and says: "Va'halo yesh she'eino mosek es zeiso ela l'achar revi'ah sheniyah" - there are farmers who only harvest their olives after the second revi'ah, so that this point is too early to permit the gifts to the poor, since the harvest is only beginning at this stage.
Rather, the point of permission, at which any person may take what remains, is "Mishe'yiheyeh he'ani yotzei v'lo yiheyeh meivi b'arba issaros" - when the poor man goes out to the field and no longer finds in what remains the value of four issars to bring home. The reason for this: Rabbi Yehudah holds that if the poor man cannot return with four issars, he gives up hope. Four issars is the measure of four meals, and four meals are enough for two meals for him and for his wife.
In summary: In this mishnah we learned the times at which the right of the poor lapses and their gifts become permitted to every person: with leket (and according to one version, with shichechah and peah as well) - once the nemushos have gone; with peret and olelos - once the poor have passed through the vineyard twice; and with olives - once the second revi'ah has fallen, and according to Rabbi Yehudah, once the poor man can no longer find the value of four issars and gives up hope.