changing the duration of each stay

I have just been issued an "F" visa of one year duration, with multiple entries, but only of 30 days each in duration. I am going to be a visiting professor at a Chinese university for a 92-day period, with this information contained in my visa application. Thus I would actually be required to leave the country 3 times during my teaching assignment. What can I (should I) do to get the duration of the visit extended?