You mean the whole Great Wall. Impossible with hours, it can take you days or months to finish it. But if you will visit a specific section, like Badaling or Mutianyu, then half a day is enough.

No, the wall is too long to tkae just days and months. Someone has done the whole thing in 600 over days. Even then, it's hard to say it's really the whole length, as has been mentioned in this article, parts of the wall lie deep in terrains and some plunge right into the sea.