My question was raised by this post.

Aliens crash-land at the Boston Airport (ie, in the United States). [...] The potential benefits of being able to reverse-engineer their tech alone is enough to propel the US to a new level of technology.

This happened in the early 21st century. After some years, the US have extracted enough knowledge from the spaceship and its passengers to create a huge technological gap between them and the rest of the world. Assume (if that makes any sense) that this gap is similar to the difference between WW2 and 2000's techs.

I guess that all this knowledge will leak at some point (intelligence services, private companies, bribes, retro-engineering of latest American devices...). So the question is,

How long before all other (developed) countries get to the same level of technology?

If you have to address the resulting political transformations too much, then this question may be too broad...

Skipping a bunch of me complaining about the complexities behind this, the answer is something in the frame of "they can keep their edge almost indefinitely if it gives them enough of an economic advantage". Short version if it costs too much to retool for the new technology it won't be adopted if the US is far enough ahead that other nations can't afford it. Look at the gap that developed between the US and USSR after the space race. It wasn't until relatively recently that Russia was able to catch up in the computer sciences through access to cheap components produced by outsourced offshore vendors in sympathetic countries.

This does assume an initial manufacturing monopoly, if they outsource production they'll lose almost immediately.

I think the best way to look at this is by seeing how long it took China to catch up to the USA's technology. 20-30 years ago, the average level of technology in China was 50-100 years outdated by Western Standards, now they have pretty comparable technologies in most areas of life; so, if you are beginning your narrative from any time period before 2000, that would be a reasonable time frame.

That said, we live in an age of hacking and surveillance where national secretes are much harder to keep than they once were. It only takes one researcher opening a bad email for someone else to gain access to an entire office building worth of research. I would not be surprised if half a dozen countries had at least some of what the USA discovered from the wreck, before they are even ready to go into production with it.

I don't think there will be any appreciable delay at all.
There are to many variables at play to answer precisely (and I assume the question will be marked as opinion based soon). But my overall line of reasoning goes like this - in this scenario the US doesn't have much head start over other nations, they do not have the infrastructure in place to produce alien technology devices at once. In short, the more secrecy there is around the alien technology, the slower the rate of adoption in the US itself, so the nation gets less benefit from it's new technology. The less secrecy, the faster it spreads.
The best use of this technological superiority, IMO, would be not trying to conceal it, but to patent it - but then you will have it produced in China and used globally the next morning (although US would profit economically the most, I guess).

UPD: the main point I see here is that initially US itself is in the position of a developing nation - they have reverse engineered alien blueprints and no manufacturing base to produce them. To use this technology, US would need to invest into overhauling its manufacturing base first. So, using this alien technology will get expensive before it gets profitable. And in order for it to get profitable you need to actually sell it, and other countries to buy it. So in this 'peaceful' scenario the whole world gets technology, though US profits.

If the technologies are going to be used only for military, US is going to have problems with its NATO allies unless it shares. I do not think there are many pieces of technology your would want to go to war with the whole world. And barring some comic-book superweapon, most military technologies take decades to integrate and implement.

The only way some alien technology can assure swift and decisive technological and military dominance is the case where it is its own manufacturing base - something like self-replicating nano-robots. And that is not 50 years worth of technological difference, it's magical philosophers stone for all intents and purposes.

Best case: Egypt got the wheel about 1,000 years after Mesopotamia. Similarly, China managed to delay European competition in porcelain and silk by centuries. Both of these gaps were before the existence of aircraft and electronic communications.

Worst case: The U.S. allowed Soviet spies to get key information from the Manhattan Project, so the U.S.S.R was only about 5 years behind in the nuclear race.

Not very long after it gets used.

There are basically two parts to technological progress. First, the actual ideas and concepts. Second, the enabling factors that allow you to actually execute on the ideas in practice.

Reverse engineering alien technology will only give you new ideas and concepts. It will not change the enabling factors. As such the technological edge gained from this would entirely be based on new ideas and concepts. Which your competition could easily reverse engineer as soon as you start using them. Or that can be copied thru espionage as soon as the competition gets aware of them.

But...

That said in the early 2000s, the China still had not caught up economically to the extent it now has, so the US and its allies would have been in much better position to actually use the new technology. If the technology really was revolutionary this would have created an economic boom within the US, Canada, western Europe, Japan, and so on. China, India, Russia, and so on would not have been in the position to capitalize on this boom.

The increased resources from the economic boom in turn would have enabled the affected countries to apply the alien technology to even greater extent. Which would have resulted in even further geographically limited growth.

The question implies that the UFO contained few decades worth of technological progress. If we assume decades of accelerated growth and then corresponding decades of effort by non-boosted countries to catch up, it adds up to "less than a century" of technological edge.

But lots of things change within a century. For example both China and Russia would almost certainly change their political course significantly if "catching up" would suddenly become that much harder. They'd seek to share the spoils by becoming closer to "the west". And if "the west" responded to these promising signs by opening up their technology, which happened with China, it would make catching up easier. Which would turn policies back to being more assertive... And so on. It generally is not useful to speculate beyond few decades.

And also...

You only mention reverse engineering but we can additionally assume that the UFO might contain limited amounts of resources we cannot replicate. Alien alloys, "magic" crystals, working devices we cannot reverse engineer fully.

This might give the US a unique "secret weapon" style edge that cannot be copied or caught up to in the foreseeable future.

Using this is probably a bad idea since you would essentially be saying that "the US is superior because I say so" but limited amounts of this would be realistic and even large amounts can work IF it is part of the premise of the story. David Weber's Safehold books do this as do lots and lots of alternate world stories. So it can be made to work. And giving your people a super secret cool toy or two might be fun.