75 hours from arrival to departure?

I have a flight from Korea to Switzerland with a stop in Beijing. I arrive in Beijing at 10:00 a.m. and in three days leave Beijing for Switzerland at 13:30. So, I have to stay in Beijing all in all about 75 hours. My question is if 72 free visa hours are counted from the moment of arrival to Bejing or from the moment of putting a stamp into my passport. In case of the latter -at worst- I would spend a couple of hours in the transit zone after arrival to Bejing or I would pass the passport control earlier.