First thing is to make sure that the F is the correct visa for whatever it is you are trying to do. A few years ago, many activities that were under the F were redesignated for other visa types. If it's a short-term study program you are doing, usually X2 is the correct visa category. If a short-term business internship or something, normally those are going under M category now.

If F is the correct category (usually something done with a connection to a higher education organization/institution where you're not a student), then the inviter should be able to give you an invitation letter that specifies the entire period. Durations of stay on an F visa are customized to whatever the inviter asks for. For US citizens applying in USA, the overall duration of an F is usually limited to 1 year. If there is any reason to believe that in the middle of this period, you might take an excursion out of China--like to Hong Kong or Korea, etc, then make sure you ask for a double- or multiple-entry visa and in the comment section, you can state why.