I caution anyone against transiting in PEK without a visa. Especially if it is a short connection. I would say 3hrs minimum. Any delays or reaccomodations require you to enter the country and use the 72hr visa free waiver. However, they will most likely only give you 24hrs since you are taking the next flight out. Miss that flight and stay past 24hrs and you will have the book thrown at you, pay a penalty and sign documents admitting guilt. I know. I experienced it personally.
Do not assume you have 72hrs. If they give you 24hrs, then that is all you have. And if you miss the flight due to weather related events such as a snow storm.....tough.