In my opinion, it will happen around 2040 with almost certainty, but probably in the form of hyperinflation out of debt, rather than actual default.
If you think hyperinflation is coming on, you should be buying as much real estate as you can right now and lock in a 30-year low rate financing. When hyperinflation hits, your debt will just evaporate away!
Are there specific reasons for that particular date?