There is a lot of evidence in our own history that civilization “peaks” last only 50-200 years. Egyptians, Greeks, Rome, Persia, Byzantines, etc.
We're somewhere between that.
I'd say US strength began in 1945 as we replaced Great Britain as the world's preeminent superpower.
Presuming we're still at our peak we're at year 74.
How much longer we will last will depend on the 2020 election.
IF Trump is re-elected, and we hold the Senate, he will be able to continue to re-shape the judiciary. That will go a long way towards keeping the US as a superpower that embraces freedom.