How many times can you use this?

I`ve been to PEK in January and used this 72 hr free transit to spend some time in the city. I`m planning to go to Russia via PEK in December and actually use this rule again to go to the city and see a friend leaving to Russia from PEK the same day. Also I will be travelling from Russia via PEK in February 2017 - my question is - how many time can you use this visa? Is it one a month/year or as many times while you`re staying there for less than 72 hrs?