How long do you think the U.S. will exist for?

I mean no empire has ever lasted forever (and I don't mean this in a bad connotation, but since we have imperialistic policies it seems right)(The British were proud of their empire and we can too). And even for countries which are not officially empires, their territories or borders change and shrink in the long run. I think our nation is at the top in terms of territorial expansion. We have been around for 232 years now. Does anyone think we will be the top superpower for at least another 100? it doesn't mean we will be off the face of the map, but how long do you think we'll retain our current size and power? It has to happen sometime, no other country has been the "king of the hill" forever. Or do you think we will pioneer this statistic?