We are planning on visitng China then visit Hong Kong then go back to China and will ask for a multiple entry visa if possible. If there is a maximum stay of 30 days in China, does this "reset" as we re-enter China or is there a maximum stay of 30 days for the duration of the validity of the visa? (e.g. Stay in China for 20 days then visit Hong Kong for 5 days then back to China for a maximum of 10 days, or is it 30 days?).

Thanks!