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.
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.
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.