How long is long? It depends on who you ask.
When I read James Howard Kunstler's The Long Emergency, I practically breathed a sigh of relief. Finally I had found someone who understood the dangers of fossil fuel depletion. Since I had read Marion King Hubbert's work in the early 1980s, I felt like I was a lone voice in the wilderness trying to warn people. At the time I shared Kuntstler's rather bleak view of the coming decades.
Many in the sustainability movement use the concept of the "seventh generation," a guideline that the Iroquois are reputed to have used. At roughly 20 years per generation, that is 140 years in the future.
I have a great deal of respect for John Michael Greer. His blog, The Archdruid Report, is one of the few things I anticipate reading each week. I find myself agreeing with a great deal of what he says. Where we differ is in a matter of perspective. Like many others in the Peak Oil movement, he sees the future as a matter of decline. Some of his most popular books are The Long Descent, The Eco-Technic Future, and The Wealth of Nature. The future centuries he envisions are not as bad as most Peak Oil prophets imagine.
Marshall Savage had a broader perspective. His masterwork, The Millennial Project: Colonizing the Galaxy in Eight Easy Steps, is a grand, sweeping vision of a bright future. Regrettably, he actually starts the book at Step 2, with Step 1, the Foundation, as an epilogue. It almost does seem like an afterthought, not having been as carefully thought out as the rest. While the dream is still being kept alive by the Living Universe Foundation, little progress has been made.
The Long Now Foundation has one of the longest time horizons I have seen. One of their concerns is the Y10K problem; they suggest everyone start writing the year with a leading zero, e.g. 02011. Another project is the 10,000 Year Clock. Their goal is to encourage long-term thinking.
Why does perspective matter? My answer to that is taken from The Rules of Victory, a translation and commentary on Sun Tzu's The Art of War. The best way to win is to "take whole", which in simplest terms could be described as getting your opponent to surrender without a fight. (It actually is much more subtle than this; I highly recommend reading this book to fully understand the concept.) As a rule, it is the person with the greater perspective who has the best chance of victory.
How long is long? For my answer, take the chart of any "peak" which isn't necessarily dependent on a nonrenewable resource, for example, population, knowledge, energy usage, prosperity. Shrink it down until it is just a small bump in a gently rising road, just large enough to be tripped over, where a human lifespan is just a point.
That is how long I mean when I say "The Long Ascent".