Long stopover in Nanjing then Guangzhou then Kuala Lumpur

You are never allowed 72 hours free transit with two stops in China. You are only allowed a maximum of 24 hours direct transit from scheduled arrival in Nanjing to scheduled departure from Guangzhou. If it's longer than 24 hours then you must have a visa in advance.

Thank you Leo. So just to confirm:

I arrive Nanjing 2:30pm and then fly to Guangzhou 7:45am the next day. Will I need to depart Guangzhou before 2:30pm, or do I have an extra 24 hours?

My problem is that most flights depart in the evening. What would happen if I stay in China for 30 hours instead of 24hours?

Don't even dream about trying to stay illegally for 30 hours. You'd probably be denied boarding on your flight to Nanjing anyway, but if you made it to China and stayed 30 hours you'd likely be detained and interviewed, blacklisted for future visits to China, and forced to buy a new expensive air ticket. Search for "Ms de Jong and Ms Watson" and see what happened to that couple from New Zealand. You must leave Guangzhou by air to a third country before 2.30pm.