Mischna zu Jeschijahu 66:23

Also he used to say that there are five things that last twelve months:The judgment of the generation of the flood [continued] twelve months; The judgment of Job [continued] twelve months; The judgment of the Egyptians [continued] twelve months; The judgment of Gog and Magog in the time to come [will continue] twelve months; The judgment of the wicked in gehinom [continues] twelve months, for it is said, and “It will be from one month until its [same] month” (Isaiah 66:23). Rabbi Yohanan ben Nuri says: “[As long as] from Passover to Shavuoth, for it is said, “And from one Sabbath until its [next] Sabbath” (ibid.).