[Dkt01]

My view, having been both a student and a mentor, is that the mentor's role is always changing; sometimes they teach by doing something and explaining the process to students, but many times they should be consultants.
My high school team had a policy that outlined a learning/teaching process: the mentors would first show how to do something, then they should have the student help, then they would assist the student, and finally, the student would work independently with the mentor as a supervisor and consultant. This process was very useful because everyone learned how to build, program, or whatever they wanted to do. Furthermore, veteran members had the independence to build and have pride in "this is our robot". Our team never did the best at competition, and we learned through many, many mistakes, but we loved that we got to build whatever robot we wanted. Design was always done by students, with mentors only answering student questions and pointing out potential flaws.
The team that I help now is focused on a student-designed, student-built robot. We mentors only provide technical instruction and assistance. I think it's really rewarding to see the students take the skills they are taught and build something that they can be proud of. There are many times that I could do something better or faster than the students because I have more experience, but then I realize that I would not have learned had I not been given the opportunity to work at my own learning pace when I was a student.
Just my two cents, but I don't think there is a time for mentors to build the robot.