And how many until the US no longer exists in its current form?
Good question. I think there will be some interesting inflection points coming up.
Individual freedom and personal liberty have been under open assault for the last several decades. We rightly objected to Bush the Lesser's domestic spying, but there seems less outrage over Obama's domestic spying (and does anyone even remember or care about Clinton's Carnivore project?).
Roe v. Wade is founded on the principle of medical privacy. Something that we are now actively wiping out. IIRC, there were 17 federal agencies looking at tracking all our medical records.
That's just on the individual privacy front. The very structure of government, the roles and responsibilities, division of authority, the meaning of law is now under open assault. If your the president and don't like a law, pretend it doesn't exist either by a policy of non-enforcement or withholding funding.
And, no. I have little hope that constitutional government will survive in the US for much longer. Far too many people have the attitude that if it's their guy doing it, it's okay. They fail to see the principles involved. If the current crop of potential presidential candidates even pays lip service to such things, it's more along the lines of they want to be the one choosing the music as the band plays on and ship goes down.
But just because our ship is going down, doesn't mean that I want to rush out and hang onto the boat anchor of the other ship that's going down either.