Non-Visa from United States, can stay for 90 days, what if I want to stay longer?

William, usually, if you apply for a tourist entry permit, you may be given one with stay duration of 30 days, but sometimes, one may be given more days, like 60 days or 90 days.
You can enjoy free entry to go there and stay for 90 days, after that, you can exit for a few days, and then re-enter HK to re-gain another 90 days. But the time gap between your exit and second entry shouldn’t be too short.