Hi:) I will be arriving to Guangzhou from London

but before i need to change flights at Chongqing and wait there 12 hrs for my connecting flight to Guangzhou. My question is the following: the 72 hrs transit will start the moment I landed on chinese soil (Chongqing) or I should apply for this in Guangzhou? In Chongquing I need to change terminals, but I won't exit the airport, so I guess I won't go through the customs. Another thing...for the 72hrs transit visa I need a flight ticket to another country...can this be a train reservation to Hong Kong for example? Or it has to be an onward flight ticket? And finally...I will spend 1 month in South East Asia and then I will come back to Guangzhou and take the flight back to London. The second time will I again be eligible for 72 hrs free transit or it cannot be more then once in a 1 month period? Thanks a lot in advance for your reply :)