How to I fill in section 2.5Longest intended stay in China among all entries ____ Days?

I am traveling to China with my family for the first time and only want to have to do visa process once! The trip we're taking in March is 10 days we leave china and spend a couple days in Hong Kong before flying home to US. If we want to return anytime in the next 10 years and could imagine spending 3 weeks there should I put 21 days as the answer to this question? Or do I just put the # of days I know we're traveling this trip?