72hr visa free transit, one-way or both ways?

Hello, I am flying from Vancouver-Shanghai-Bangkok, holding a valid Cdn passport, and flights all booked. If I understand correctly, I will be ok to get the 72hr free transit in order to stay in Shanghai for a day(one night hotel). I am wondering if when I am flying back thru Shanghai, 3 weeks later if the 72hr free transit will still apply? Will I still be eligible to use this 72hr free transit for a one night stop over in Shanghai on my flights returning to Vancouver via Hanoi? Thanks!