I believe the maximum time in gehenim is 11 months.
In fact, now that I think about it, I’m sure it’s not true at all.
The reason why a person says kaddish for a parent for only 11 months is because the maximum time for a “normal” person is 12 months, not 11. During the last month kaddish is not recited so as to not give the impression that the son thinks his parent was a total rasha.
That being said, I’m sure that there are those for whom even 12 months is not the maximum. In fact, I know one such person who has no chelek in Olam Habah (which would imply an eternal punishment).
The Wolf