Is there a limit on the number of total days per year I can be in China on a M business visa?

I have an M type with multiple entries, maximum 60 days per entry. I typically travel to PRC for 2 weeks per month to attend business meetings. In total I have been in there almost 70 days this year through the end of May. In other countries (such as Brazil where I have traveled frequently), I had a limit on the number of days per year I could remain in the country (my limit in Brazil, for example, was 180 days). Are there similar restrictions on business M types in China? I had heard someone say 90 days, but other statements seem to contradict this -- most saying there is no limit as long as I leave within 60 days after arriving then re-enter again.