According to Wookieepedia, only 3 years were in between the destruction of the first one and Episode 6. In episode 6, it was almost done and functional.enter image description here

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    Doesn't look almost done in that picture. I think there's something (maybe EU) that indicated the Death Star II was already under construction before Death Star first fired.
    – HorusKol
    Jan 12 at 6:29