Here's How Long It Really Took To Build The Death Star
If you were to ask a random person to name the most memorable fictional weapon of mass destruction, the answer would probably be the Death Star.
Sure, there are far more destructive things even within the strict confines of space opera – in fact, the sequel escalation led to the introduction of superweapons that completely dwarf the Death Star in both the old Star Wars Expanded Universe and the new Disney canon.
But it combines sheer power, menacing presence, and role in the plot in a unique way.
Unsurprisingly, fans are often obsessed with trivia related to the Death Star, from its actual size (the figure of 100 miles (160 km) in diameter arrived at by analyzing the movie frames eventually became official), to its firepower (if you're interested, the minimum amount of energy required to destroy an Earth-sized planet is roughly equal to the total energy produced by our sun in a week), to the length of its construction.
The latter was a matter of some debate until the release of Episode III, at the end of which we see Darth Vader and the Emperor observing the skeleton of the first Death Star under construction.
Since then, the generally accepted explanation is that it took about two decades for what would become the Galactic Empire to build the first Death Star.
The discrepancy in construction time with the second Death Star, which was made operational (though not yet mobile) within 4 years between Episodes IV and VI, despite being much, much larger, was explained by the common sense idea that the construction of the first Death Star was kept completely secret from the galaxy.
Imagine the scale of the Galactic Empire that not only managed to build such a large and complex battle station, but did so without alerting its population to a project of such magnitude! (If you can't, don't worry, neither can your typical hired Star Wars writer).
The events of Rogue One, particularly Jyn Erso's age at various points in the film, roughly confirm this timeline. Specifically, she was 15 when she was separated from her father Galen, 13 years before the main action of Rogue One, and the construction of the Death Star was well underway by that time.
So the timeline seems solid... until we do some more analysis and realize that the Death Star structure we see in Revenge of the Sith is very unlikely to be the same Death Star we see nearing completion in Rogue One and Episode 12 of Andor.
Most obviously, its superlaser dish was not built separately from everything else. Whether this was a prototype that didn't work, so Galen and Krennik had to rework the old Geonosian designs (which existed all the way back in Episode II), or whether Disney just didn't care about consistency, we can only guess.