Is there a limit on how often I can obtain the 72 hour visa?

I'll be flying from Los Angeles to Beijing to Bangkok. I have an 8 hour layover on the flight from LA to Bangkok in Beijing. I understand I can get a 72 hour free transit so I can do a quick trip into the city. I want to know if I'll be able to do the same on my return flight to LA which also lays over in Beijing for 8 hours?