How long will it take to get off flight, get 72 hour transit visa and leave Beijing airport?

It depends on the airport itself. If there are many passengers, you need to wait more time. Generally, I think you can get your luggage in 30 minutes if it comes to the airport with you on the same flight. It's very quick to acquire the 72 hours free transit. You just follow the signs to reach the windows, submit your passport and air tickets and then they just give you a stamp on your passport. It takes just a few seconds if we exclude the queuing time. In my view, you can get all these things done in one hour if the airport isn't very busy.