Chapter Two, Mishnah 10 - the closing mishnah of the chapter, and it too comes from Rabbi Akiva. Once again we have a list of five items, and this time: "Af Rabbi Akiva amar" - five periods whose duration is twelve months.
The five periods that last twelve months:
The judgment of the generation of the Flood - it lasted twelve months. The verses record it from the seventeenth day of the second month until the twenty-seventh day of the second month of the following year, which appears to be ten days more than a year; but the solar year, as against the lunar year, is three hundred and sixty-five days, ten days longer, so that what we have here is exactly one complete year.
The afflictions of Job - that whole episode of his illness and his suffering lasted twelve months, as emerges from the verses.
The judgment of the Egyptians - the period we call the Ten Plagues lasted twelve months. There is the well known Midrash quoted by Rashi that each plague lasted a month; and since ten months are not twelve, we must say either that no plague lasted exactly a full month, or that there were intervals of time between the plagues. Either way, the total came to twelve months of punishment for the Egyptians, in the year preceding the Exodus.
Gog and Magog - the process of exacting punishment from the nations that wage that war at the end of days against the Jews in the Land of Israel will last twelve months. This does not mean that the war itself lasts twelve months - it ends more or less on that very day - but rather that the sheer number of corpses lying about is what requires a period of twelve months.
The judgment of the wicked in Gehinnom - it lasts up to twelve months, and the source is a verse in Yeshayahu.
The wording of the verse is: "vehayah midei chodesh bechodsho" - from month to month, meaning from a given date until that same date in the corresponding month of the following year. But the verse continues: "umidei Shabbos beShabbato" - one clause speaks of a month and the other of a Shabbos, and from here we learn that it is that very same time a year later.
Yeshayahu the prophet is prophesying here about the return to the Beis HaMikdash and to the service of Hashem, but the derashah sees in serving Hashem and bowing before Him also part of the process of Gan Eden at the end of it all. In any case, the point that emerges from Rabbi Akiva's words is that the judgment of those sentenced to Gehinnom lasts twelve months.
The view of Rabbi Yochanan ben Nuri:
Rabbi Yochanan ben Nuri derives his ruling specifically from the second half of the verse, "umidei Shabbos beShabbato", and understands that the count runs from Shabbos to Shabbos. This "Shabbos" carries the same sense as in the passage of the counting of the Omer, "mimacharas haShabbos", where "Shabbos" refers to Pesach. It follows that we are speaking of the period from Pesach until Shavuos, seven weeks.
That is why Rabbi Yochanan ben Nuri says: "min haPesach ve'ad ha'Atzeres" - the period of judgment for the souls sentenced to Gehinnom is only the seven weeks from Pesach until Shavuos, and he brings his proof from the conclusion of that very same verse: "umidei Shabbos beShabbato".
In summary: in this mishnah Rabbi Akiva lists five periods that last twelve months - the judgment of the generation of the Flood, the afflictions of Job, the judgment of the Egyptians, the exacting of punishment from Gog and Magog, and the judgment of the wicked in Gehinnom. On the last of these Rabbi Yochanan ben Nuri disagrees, deriving from the end of the verse in Yeshayahu that the period of judgment lasts only seven weeks, from Pesach until Shavuos.