Approx 6 hours. What good is tide in the open ocean? The only place you could harness one is where tides cause currents through narrow passages. Sure, you could do that, but there aren't that many sites, and you still have to solidly anchor the machine (expensive). Then of course is the difficulty with maintenance in corrosive sea water, and the difficult of servicing such a machine (the tides don't stop for maintenance!).
Shut the fuck up. You know nothing about this at all.