How long must one be logged out for bonus turns? Is it still 12hrs for 3 turns and 18hrs for 6?

yes but no need to be away for 18 hrs, you can log in after 12 hrs since last time you got bonusturns (and as many times as you want between those 2 times) and then 6 hrs later to get another 3 turns (same as before).

what marshal is saying is there is 2 timers, one for the 6 turn and 1 for the 3 turn.