It's a political question. In principle, the USD can collapse in value but still remain in existence.

Maybe the big governments will file for bankruptcy, as Greece did. That may somewhat put a lid on inflation. Possibly they won't be able to manage it and start inflating crazily. Then, the question is how many people cope with it and how many start using more stable currencies or commodities.

I guess the end of the USD will be at least a good bit after it has lost most of its current value. Therefore, the exact date isn't all that important when determining how many dollars to hold on the long run.