I dont know about 3rivers specifically, but if they say minimum, they mean minimum, not limit or maximum. some programs offer fixed lengths, others open ended, and at least one has a fixed with a fixed (somewhat different) extension option available.
I think good wilderness programs are highly valuable, but are not for everybody. Some programs ought not be for anybody. But before you even get to looking at a specific program, you ought to answer the questions already posted:
" What is the kid's problem/problems?
What steps have you taken prior to arriving at this point?
What sort of background does the child have? "
Then ask your questions about stay, safety, staff certifications etc OF THE PROGRAM FIRST, then maybe check the program's answers against what you get here and elsewhere.