how long will this game take to complete?, will it take at least 12 hours?
It depends a lot of your play-style. If you want every building repaired, everything bought, each single mission played up to 100 %. Then you'll certainly have your twelve hours (without a doubt).
if i like to explore a bit but stick mainly to the story missions will i still get 12 hours
From start to finish if you settle for 50% sync and dont worry about secondary objectives, doing nothing but primary memorys (taking into account a few failures and toilet breaks) it will take a seasoned player of assassins creed baring in mind it will be your first playthrough. Even if you have the time to go for a single marathon sessions it will take around 8hrs perhaps, if its your second playthrough and you know the layout and have adapted to the controls and other such waffle (including skipping the cinematic trailers) would say going for same objectives less than 6rs of course these figures arent in any manner a precise time, more an educated guess...
But hey goodluck to you
Forgive my typos, I have a newborn baby and at the moment even getting straight out of bed I still feel asleep