I just apply for Hongkong visa today, how long can I have it?

Did you apply from the Chinese embassy in Japan? Did you receive the pick-up receipt from them? The receipt should tell you when you should go back to embassy to pick up your application. Generally speaking, if your application is handled by Chinese embassy, then you can get it in four working days. If your application is sent to HK for approval, then you need wait at least four to six weeks.