This question already has an answer here:
Inspired by this question, how long did it take to build the original Death Star? Much of the dialogue in Episode IV indicates that the battle station is not "operational", but no mention is mead of how long it actually took to construct the station.
One could deduce that it took 19 years to build, since the construction begins at the end of Episode III and is only just operational near the beginning of Episode IV, which is 19 years later. However, it is equally as possible that the station has been built and ready for a while, but has simply not yet been used.
How long did the actual construction of the original Death Star take?