Normally the duration of stay of a China tourist visa(https://www.travelchinaguide.com/embassy/visa/tourist.htm) is 30 days, if your estimated stay in China is longer than 30 days, please make it clear in the application form and ask for the Visa Officer's approval when you submit your application. it is up to the mood of visa officer, if he is very happy, you can get a double-entry tourist visa with each stay duration 90 days.