Nothing lasts forever, and the United States is no exception. How long can we expect it to last?
To give some context to the question, a long look at history, going through China and Egypt (see chart below) and MesoAmerica suggests that humans have considerable difficulties establishing geographically large and well-populated political entities that span for longer than 5 centuries. No Chinese dynasty seems to have even made it past the 300 year mark in 2 millennia. In fact, there are only a handful of successful examples in the entire history of humankind (If we restrict ourselves to larger entities, I can think of Portugal's colonial empire, the Ottoman Empire, and the various incarnations of the Roman/Byzantine republic/empire, with more smaller scale entities listed here that I would probably (unfairly) hesitate to call empires.
Now, let us consider a few facts. Technology has enabled faster flow of peoples, but populations are correspondingly larger. Human lives are longer, but also more fast paced. Technological levels are unprecedentedly high, but that might make modern societies more fragile to disruption. The US has nuclear weapons, but also humanitarian drives that, say, the Romans lacked towards what they considered 'barbarians'. The United States is the strongest political entity in the world economically and has been in this position for what is over a century. The United States is the strongest political entity in the world militarily and has been in this position for almost 3/4 of a century now. The United States has a centuries long tradition of incorporating immigrants into its body politic and gaining their loyalty. The United States has been in existence for 2.4 centuries already.
Given all that, how long can we expect the United States to last? If you think providing date-like timelines is too hard, I will also consider a discussion of the possible factors that will eventually bring down the Union as an acceptable answer.
The question of how one defines the end of an empire/political entity might be relevant, so I'd hazard a soft definition including external conquest, civil war resulting in dissolution, loss of the imperial heartland or of the vast majority of its territory to other entities, or a combination of persistent and irreversible demographic, economic and military setbacks that render the entity irrelevant on the world stage. For instance, a Byzantine 'empire' restricted to an area of a city state was centuries past being effectively an empire.
EDIT: After extended discussions in chat, for the purpose of this question, we'll restrict the definition of the end of the USA as failing to keep all of the following: 66% of its present territory AND (66% of its population OR at least 2.5% of the world's population), AND 66% of its current constitution, excepting future growth through the amendment process.
Edit 2: I use the term "empire" to mean geographically large and well-populated political entities. This is hard to pin down exactly, but say at least 1% of the world's area and at least 1% of the world's population.