In Roman days they walked or rode donkeys. Now we have instantaneous communications around the world and are testing an airplane that goes 13,000 miles per hour.
Everything is moving a lot faster and I expect our decline will be faster. I'm guessing a couple of generations, especially if we keep electing people like we currently have in office.
The Eastern Roman Empire, aka the Byzantine or Greek Empire, managed two major ascendants between its foundation by Constantine on former Byzantium (which had been destroyed for rebellion by the Severi, decades before), under Justinian and Heraclius. Its real "beginning of the beginning of the end" began with the Second Battle of Manzikert in 1071 (not much of a battle, really -- but the Turks were left in possession of the battlefield, and the Greeks abandoned the adjacent territory, and the Turks' cutoff of Christian access to Jerusalem began the Crusades 20 years later). The accelerating decline began with the sack of Constantinople by the Franks of the Fourth Crusade in 1204, and ran to extinction in 1453.
From 330 to 1204 is a very long, successful career for a single state; and it managed to hang on for another 250 years after that.
True. Besides, we haven't had our 5 Good Emperors century yet (trying to make lemonade here).