Length of time in Third Country; Connecting Flight Back to Originating Country

Hi,
Do you know if there is any requirement with the 72 hour TWOV for how long you must stay in the third country. For example, could someone fly to Beijing from country A, stay in Beijing for 2 nights, then fly to country B but only stay a few hours, and then return to country A the same day?

With the above scenario, must the person go through immigration and go out in country B and then come back through immigration in country B, or could they simply remain in the airport?

Thanks.