Dear sir, I need your Guidance.

In regards to the invitation sent to me via my email, from a company by name Guangdong Forstar Precision Machine Co.,.Ltd from China this day 13th July, 2016 for a 2 weeks Agency Training in China.
I Obayagbona Caspar Pedro, a Nigerian, with international passport number: A03786434.
With due respect sir, i wish to request for your guidance.

1. In regards to the above, what is the minimum duration of Visa that can be issued to me?
2. Is there any need for the company that sent me the invitation letter also send a copy of the invitation letter to the China Embassy?
3. Why is it that the company inviting me sends them (China Embassy Consular) copies of the invitation letter through Fax system, no respond?
4. Copies through Email services no response too. Why?

Best regards,
Caspar P. Obayagbona.