Limit of how often the 72 hour visa waiver can be used?

How often can I take advantage of the 72 hour free transit? Is there a limit? I have flight from Sweden-Beijing-Ulaanbaatar with a stop over of less than 72 hours. That should be OK, right? But, heading back Ulaanbaatar-Beijing-Sweden I have 23 hours 50 minutes layover in Beijing. Can I use the waiver again for this case?

Best regards,
Mikael