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.
That's my take on it as well. It MAY take longer than I'll live, but just in case . . .
Bingo. Saved me the trouble of typing my answer. In 100 AD, it took a few weeks to get a message from Jerusalem to Rome. Today it takes as long as it takes to hit "send".
I have a personal theory which is the rise of the English speaking world was largely attributable to the language. While English can be exceedingly difficult to learn due to the size of its vocabulary and it's variabe rules, it allows for the far more precise and exact communication of ideas. If a short conversation between two people takes 15 seconds in English and 18 seconds in German, Spanish, Portugese, etc. the English speakers have increased the efficiency and decreased the time required for the exchange of data...it's really no different than increasing processor speed on your computer. When you multiply those few seconds savings across every conversation held in English among millions and millions of people, over hundreds and hundreds of years, it makes sense why the English speaking world quickened the pace of technological advance over other nations.
As you say, things happen more rapidly in our world, and I suspect that when things start to collapse, they will do so, not over the course of centuries, but within the course of one, or at the most, two lifetimes.