Till the government pulls the plug on alternative "currency."
The minute any government sees these crypto currencies as a threat, they can declare them illegal within their borders, and people will end up with just files on computers.
The US Constitution grants only the government the ability to produce money. As soon as they decide to crack down on alternate currencies, the stuff becomes worthless.
Actually the Constitution is pretty specific about what money is, and FRNs are not Constitutional money.
Art 1, Section 10:
No state shall enter into any treaty, alliance, or confederation; grant letters of marque and reprisal; coin money; emit bills of credit; make anything but gold and silver coin a tender in payment of debts; pass any bill of attainder, ex post facto law, or law impairing the obligation of contracts, or grant any title of nobility.