How long would it take for a student visa to be change into working visa in China?

It is very hard to change a student visa into a work visa within Mainland China, if you have found a job, you can ask your employer to get the Alien Employment License issued by Ministry of Labour and Social Security or Foreign Experts Working Permit issued by the State Administration of Foreign Experts Affairs, the official invitation letter issued by Foreign Affairs Office, you have to use these documents to apply work visa in your home country, the physical examination record is also required.