score:18
No definite figure can be given, as so much depends on circumstances.
Given enough people, premade parts, and proper tools, a trebuchet or similar device (and this has been done by historians and archaeologists: it was aired on History and I guess elsewhere a few years ago, but I'm sure there are more sources) can be put together in a few hours by a relatively small team.
Upvote:11
The 'Highland fling', a trebuchet project in Scotland took some two weeks and around 35 people.
It worked.