Believe it or not, I totally can buy that it took 20 years to complete the first Death Star. If you ever look at how the military brings a new weapon system from the drawing board to the front lines, it's a long process and takes about the same amount of time.
Plus, dealing with a prototype, there's going to be all sorts of unforseen problems which slows down the production. I could see this being even worse with a moon-sized space station! Once you get the first one done, making subsequent modaels are much easier, since you have learned to overcome most of the obstacles making the original.
I could also see that the Emperor would want a bunch of Death Stars. These things aren't exactly the fastest moving vehicles in the galaxy, and you'd really need a bunch of them to keep the rebellious star systems in line.