90 day limit if you hold a Chinese multi entry M-type (business) visa.

Can you tell me if there is a 90 day limit per calendar year on time spent in China if you hold a Chinese multi entry M-type (business)?
I understand that the length of each stay is determined by whatever is granted on the entry permit (30 days, 60 days, 90 days, etc) - I'm interested if there is any cumulative limit of 90 days in a calendar year. For example, if you did 4 stays of 25 days, you would have spent a total of 100 days in China.
One rumor I had heard was that there might be a limit associated with the Chinese department of labor, and not with the visa itself.
It would also be great it I could get any primary source reference, like the specific law that decrees the restriction (if there even is one).
Many Thanks,
M