90 day duration-2 entries....when do I need to exit by for trip to HongKong?

I have a Business-F for a duration of each stay of 90 days. I have 2 entries. My enter before date is Oct. 9, 2013. I plan to enter on May 6th, 2013 and I have a flight booked back to Canada on Aug. 1st, 2013. I am doing research in Hangzhou while there.
I want to do one weekend trip to Hong Kong, which requires an entry, and was hoping to do it mid July. I am also doing other trips to Shanghai, Beijing and Xian(these destinations do not require an entry correct?) during my 90 days.
I am being told by a traveling mate that I have to use the Visa to go to Hong Kong within 60 days of entering China, so this would mean I have to leave mainland for HongKong by July 4th(60 days after arriving on May 6th. I thought I just had to use the entry before the 90 days? Which is it it? If I only have 60 days to use my 2nd entry, this will affect when I book the weekend to Hong Kong. If I have 90 days to use the entry, then I can do the visit closer to the end of July. Would these entry conditions be the same on Student and Business F?