Business Visa (F Visa) is issued to an alien who is invited to China for a visit, an investigation, a lecture, to do business, scientific-technological and culture exchanges, short-term advanced studies or internship for a period of no more than six months.  A business visa is required if you intend to participate for up to 6 months in business activities as an employee of an overseas organisation; investigating & establishing a business activity; lecture, study, or attend training (including internships). In some situations overseas businessmen with a need to travel frequently in and out of China over a longer period may be granted a 12 month visa.  Normally, F visas are given for a 30 days stay, if you need longer stays, must clearly indicate so in the visa application form, and hope the Embassy Officer will grant it.  Normally they will not grant a large increase.

To obtain an F visa you must have an invitation letter from a Chinese organization that you will be dealing with or a reference letter from your employer explaining the purpose of the visit. The inviter should check that their invitation complies with all regulations.

The business visa (F) has over the years been used as a 'catch all' option. Visa issuing offices seem to have been willing to issue them relatively easily and often for longer durations than a tourist visa. It is not permitted to work as an employee of a Chinese business in China while on an F visa (there are a few exceptions such as working for a company funded from overseas), but many people have done so, often with the full knowledge of local officials, but on the other hand some people have been expelled!

Evidently you are not too familiar with Chinese visas.  In them there are three things (depends on type of visa) that controls your visit, and they are:  validity, number of entries and stay.  The validity of the visa is the window of time in which you will be allowed to enter China, and normally is between 3 and 24 months.  Number of entries is self explanatory, normally is one, two or multiple entries.  Stay is the number of days allowed in China each time you enter.

So, an F 3 months allows you to enter China during 90 days any time you want but within the number of entries.  F 6 months does the same but for six months in time.

Excellent breakdown by HappyExPat.
I would just add that your F visa type will be determined by the local government when your inviter submits the paperwork for the request.  The first time I got my F visa (already had 4 L visas before) they only gave me a 3 month 2 entry 30 days at a time (and my inviter requested a 2 year multi-entry 60 day).  After the second request and some tweeked request paperwork, they have given me a 6 month multi-entry 30 days.....Yeah, now I can go to Hong Kong every weekend.
It will all depend on what the letter from the government says you are intittled to, as you need this letter to apply for the visa.  It seems like it is a progressive thing, and I hope to land a 1 year multi entry 60 day F visa next time.

Yes, I know what one poster above has written comes directly from a government site but that being said,

it is possible to convert a residence permit to an F visa in country but generally closer to the end of the term of the residence permit and you may wish to check with this agent if you are in China.  I have dealt with them for several years for others.

http://www.visainchina.com/fvisa.htm

Additionally, if you want the best visa agent in Hong Kong and at a reasonable price, then you should try this agent:

http://www.fbt-chinavisa.com.hk

I have found them exceptionally helpful in the past.

Dude, thanks.  It has been my expeience in getting "L" visas for almost 15 years that Officers at Embassies abroad do not grant to first applicants visas with "good" terms for whatever reason, and also do not grant large increases in the next one or two rounds, they sort of give you small increments on each round. 
When I first applied, I asked for a one year visa, multiple entries, 60 days each, and got a one entry, 6 months, 30 days stay instead.  Second time was still 6 months, but two entries, 30 days each.  Then they gave me one year, still two entrries, but 60 days each time.  Next was one year, multiple entries, 60 days.  Then I graduated into 2 years, Multientry, 90 days, and I am now on my second 2 years, multientry and asked for 180 days stay, got 120 or 150, do not recall of the bat.

My suggestion to you is to write an explanatory letter explaining your situation in detail, using often words as "please", and "it would be more convenient", "allowing me more time to train others", etc.  In your letter, also detail how many visas you have already had, and point out always following instructions while in China, since you have respect for them.  It helped me get what I wanted, maybe it will help you too.

Thanks for the answers guys and thanks for the agency contact info in Shanghai.
So there is no F Visa 6 month with a 6 month stay? You have to leave every 30 or 90 days?

Yes, you can "stay" as many days as granted when the visa is issued.  As I said above, it starts with a normal 30 days, you may get 60 with a good convincing reason on a first time basis.  With the next visas, you may get longer "stays" if requested, but must offer a good reason for requesting longer stays to increase chances of getting it.

But before I leave China to return home to get the visa, I would contact the visa agents in Shanghai and see if they can get you the visa without leaving China and the cost, maybe it will be cheaper than flying back home.  Be careful you do not end on a scam either, please.

Fabian, it is my understanding that the length of the stay is in most cases tied to the nationality of the passport holder.  Long-term "F" visas are available with longer stays except that as of this moment, they are not available to citizens of the United States nor to citizens of France.  Reciprocity issues.

If you check the Shanghai visa agent's site, there is a chart there somewhere that will tell you how long for each F visa and how long the can be, depending upon country of origin.

Ok thanks guys.
Ill let u know how it went!

So this is the result:

I will get a F Visa for 3 months because the rules are stricter and 6 months is difficult to obtain.
Shortly before the 3 months expire I can extend it to another 3 months in the visa office in Shanghai. Sounds too good to be true but I heard it from 2 different companies.

Fabian, I am happy to hear your good news.  You may be limited to a 3-months' visa because of nationality.  At present, Americans and French are being given only 3-month F visas.  Still, all-in-all, it should give you some hope.