Production time is 13mins 9secs + travelling time. Assuming things don't change when it goes live, cost is 20K gems & it produces 40K gems over a one year period.
I was a little suprised to see there doesn't appear to be a limit on the number of gem mines you can purchase (other than the size of your wallet, I suppose). I managed to build two on the test server & I presume I'll be able to build two more after 6 months are up. Of course, the test server is likely to be wiped by then, so I'm only guessing