C. S. Lewis thinks this is not clear.
“I notice that Our Lord, while stressing the terror of hell with unsparing severity, usually emphasises the idea, not of duration, but of finality. Consignment to the destroying fire is usually treated as the end of the story—not as the beginning of a new story. That the lost soul is eternally fixed in its diabolical attitude we cannot doubt: but whether this eternal fixity implies endless duration—or duration at all—we cannot say” (PofP, chap. 8, par. 10).