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Is the time a turn takes ever mentioned?

It's the time to fire and reload a bunch of torpedos or assault craft.
It's the time to move 20-25cm on the table top. No clue how far that is though.

Due to the vast distances involved with 40k's interstellar combat, the duration of space battles is somewhere on the scale of hours to days. So a single "turn" in BFG could easily represent several hours of real time (keeping in mind that with IGOUGO systems, your player turn represents all actions taken by your forces throughout the entire game turn).

So a turn is about an hour to several hours?

Sounds about right to me

Never defined in the rules - primarily as it's irrelevant.