The estimates that I have seen assume an arbitrary cutoff - say, at 2050. That said, it doesn't much matter what they do since the future value beyond about 30 years is so small and the total liability is so absurdly large.
The one future budget number I find reasonable, and scary, is the expected federal income at current tax rates versus the future estimated expenses. Those numbers are all over the place but they suggest we won’t be viable in a few short years. Our rate of freebie expense growth is outpacing our income growth by a large margin.