12 hours seems generous to me, just to remove and replace a head gasket. If you have done this before and have all the tools/parts you should be able to throw a gasket in faster than that.

Just buttoned up my 240D yesterday after a blown headgasket. It took a looonnng time. But was my first time taking apart one of these engines. I didn't know what I was doing with somethings and was terrified of messing up the timing. I even dropped the woodruff key into the engine!

But knowing what I know now I think I could tear one apart and re assemble fairly quickly.