When does 72 hours finish - by leaving the city or leaving the country?

I'm coming from Taiwan, stay in Chengdu for 72 hours and will fly out to Singapore. But, my flight from Chengdu to Singapore has another layover in Shanghai. Do I have to include this layover time into my 72 hours? Or does it finish when I leave Chengdu for my international flight to Singapore? I do not intent to leave the airport in Shanghai.