I have heard from the mashgiach of Kaminetz in E”Y (r’ moshe aaron stern zt”l) about this topic, a few times, and he quoted seforim, saying: There is an idea that Gehennom is for a MAX 12 months – no more. After that, the soul goes into GanEden clean and pure. However, that’s only if a sinner MERITS going to Gehennom. A Rasha Gomer is much worse off, his soul gets flung from one end of the world to the other ETERNALLY – whatever this concept means but is excrusiatingly(?) painful and the suffering is unbearable.
There is a story about a big Kabbalist, he quoted, (I forget his name, he was a Sefardic Kabbalist)who lived about 100 years ago, was once approached by a soul who begged for him to daven for him. He was one of those “wandering souls” that found no rest, and has been suffering for years and years. He asked him his name and asked him what he had done wrong in his life. He told him that he was the FIRST roman soldier to fling a stone at the Bais HaMikdash by its destruction. The Rav was stunned. All these years he had been tortured by the malachei chavala, to no end. He begged for a tikun to be “accepted” into Gehennom. He agreed to daven, and after a week or so, the soul came down and told the Rav, thank you, he was just admitted to Gehennom! SO AT LEAST THERE WOULD BE AN END TO HIS SUFFERING!
Oyom V’Noyra!