the talmud in Rosh Hashana 17a lists some sins which warrant eternal gehinom.
Is this to be taken literally? or perhaps it means a very long time or until the end of this world.
Rabbi Yonasan Hirtz, rabbi of Utopia Jewish Center, in Queens, lectured once that the rabbis' explanations of what happens after death is pure speculation. "No one knows what happens after death because anyone who knows is dead," he said. The purpose of the speculation, like many midrashim, is to help us prioritize our values. One might believe that the rabbis' views were traditional teachings that Moses received at Mount Sinai and passed on as part of the oral tradition. But I think not. The Gemara at Rosh Hashana 17a (and the discussion before and after that page) do not specifically cite to such a tradition (although they do not always say), but rather use references to the words of the prophets to support their points.
For example, the Gemara at Rosh Hashana 16a makes reference to a prophecy in Malachai 3:21 ("And you shall crush the wicked, for they will be as ash under the soles of your feet on the day that I will prepare, says the Lord of Hosts") to support the position that the souls of the wicked are sent to Gehinom (Hell) for 12 months after death and then reduced to ashes in the flames, scattered in the wind, then becoming the dust that the righteous walk upon.