10 Year M Visa Questions.

Hello I entered China with a business M type on February 21st, 2016. The duration of stay is 60 days so I would like to know if it would okay if I exited PRC on April 21st, 2016. That will be the 60th day but I'm not sure if that will be to long. I really do not want to overstay so would it be better if I left PRC a week earlier than April 21st. If I leave on the 60th day will I be questioned by customs officials when I leave?

I also have another question. What's the maximum total length of time I can stay in PRC on a ten year business M type? Like if I were to stay a total of 10 months spread out over 1 year (exiting every 60 days)? Is there anything preventing me from doing that? Is there a maximum period of time I can stay in PRC while on this M type?

Thank You.