I think you might have a misunderstanding. You don't need to leave the country every 30 days with a 6 month visa. You need to leave the country only with an L visa, which is the tourist visa, but not if you have purchased a 60 day visa. If you purchase a 60 day visa, then you need to leave China, presumably for HK, and buy a new 30 or 60 day visa OR go to the visa offices in China and have the 60 day visa EXTENDED but its only allowed to be extended for 30 days more. After that 90 days, you absolutely must go to HK to get a whole new visa, usually another 60 day visa which you would extend in the mainland for 30 days after the 60 day period!!

Now, that was being done rampantly for the last ten years, but i was in HK in Nov. 2014, and while in the hostels, i found out that the Chinese visa offices in HK were told to stop renewing all of the repeat tourist visas. So, I guess you can buy one additional, but after one, they reject you, etc. And then, you cannot get the visa in HK anymore and they tell you that you must return to your own country, etc. as my friends told me at the hostels.