As it was said earlier on of the other post, Lucas specifically states in the ROTS commentary with a chuckle, that he was aware it took the death star 20 years to build, but he said they had union problems and stuff like that.

I always wish that in ROTS they would have shown 2 death stars being built. So it would make it seem that maybe after 5-10 years, they decided on just finishing one to get in up and working, which would leave it plausable for them after the first was blown up to continue on an already built second one.