TL; DR: How long can bitcoin's purchasing power go? I have seen pundits posting as high values as 1 satoshi == $1. I do not think this is possible, the realistic limit for a value of 1 bitcoin is $300,000 in 2013 dollars.
I think the largest possible long-term value for a bitcoin is reached, when all the civilized world is using bitcoin as the reserve currency, upon which the international trade rests.
As we all should know (albeit very few do), gold was used as a sole reserve currency after the hijacking of bimetallism in about 1873. The gold standard was suspended in 1914, following the outbreak of the Great War. The framework for its destruction was already started being laid in 1913, when the Federal Reserve was founded. The destruction was carried out piecewise following World War I, so that FDR in 1933 could blame the remaining vestiges of the gold standard on all of the world's problems, and order the gold confiscation of 1933, and the devaluation of the dollar by 40%. This caused the great depression of 1933-1941. They needed another war to lift the country from stagnation. We are in a similar precarious situation now, and an apocalypse can likely be avoided, only be quickly increasing bitcoin's value, which is why I post this essay.
During the gold standard, as well as
both before and after it, the total stock of above-ground gold in existence, divided by the population of the planet, has stayed remarkably constant.
In 1913, there was 1.65 billion people and 31 000 tons of gold. It is 18.8 grams per person.
In 2013, there is 7.11 billion people and 165 000 tons of gold. It is 23.2 grams per person.
Gold's purchasing power has stayed essentially unchanged from 1913 to 2013. A skilled worker in the U.S. earned $50 per month, which is 2.5 ounces of gold. The same 2.5 ounces today costs $3,500, which is still a reasonable wage of a skilled worker. Both amounts let you live in a relative comfort.
In 1913, all international currencies were defined in terms of a fixed quantity of gold, and were therefore linear representations one of another. Despite the large number of bills (banknotes) in circulation, the existence of
real bills kept the gold's value from appreciating without bounds, regardless of the fact that the world trade and living standards increased very rapidly from 1873 to 1913.
When there was many goods coming into the market, the increase in the economy did not require dipping into the pool of the available gold coins. On the contrary, the overabundance of goods effected the rise in discount rate, which made holding
present gold a relatively worse deal compared to holding
future gold (i.e. real bills denominated in gold and payable in gold coin only, after a maximum of 91 days from writing,
with 'interest'). Therefore the inordinate rise in gold's price as the consequence of specialization and the economic growth never happened. If somebody says so, it is not true. The economy expanded on self-liquidating, non-fractional reserve credit. (There was also non-self liquidating credit available such as mortgages and business loans, but delving into them is not the issue here.)
Bitcoins could serve as the basis of the future economy, in the similar way as gold did in 1873-1913. As as little as about 20 grams of gold per person could serve as the economic and financial backbone of the planet in 1913,
BTC0.003 will do in 2013 if we give it a chance.
Making a quick conversion, using the value of a U.S. worker's monthly wage, we can calculate the potential value of bitcoin if this scenario ever becomes true. The monthly wage of a skilled worker would be 1.2 million satoshi, having a purchasing power of $3,500 in 2013 U.S. dollars.
Therefore a satoshi will be worth no more than $0.003, and 100 million satoshi = 1 bitcoin, will be worth $300,000. Any price above this (remember to factor in the USD inflation, though!) is likely a short-term aberration, or a bubble, and should be treated accordingly.