There are only two times when you can leave a job and not have it affect you adversely. For one, you are there for 2-3 years and you move on. No problem with that. In the case of a postdoc, you leave after your contracted term ends.
But, "sooner is better" on problem employment. If you have these difficulties now, they are only going to get worse. They aren't going to ease up. This person has shown his stripes. NO, a 12 hour day is not normal. Many people will work that (myself included) but it isn't necessary to be successful. And no boss should ever demand it. There are many reasons you may want to leave.
I'd get out of there and go immediately to work in another lab -- I wouldn't take months of extra time off on top of this departure which will already show as a hole in your CV employment timeline.
“I shall tell you a great secret, my friend. Do not wait for the last judgment, it takes place every day.” - Albert Camus