At what point does it become unacceptable for a mentor to design/build the robot

[Chris is me]

It's unacceptable when students aren't inspired anymore. Anything less is a cop-out.

I've seen students inspired by 90% mentor built robots and I've seen students turned away from engineering forever by 90% student built robots. I've seen robots that look "mentor built" that come from excellent programs with students inspired at every level, and I've seen robots that look "student built" that come from terrible programs with almost no actual student involvement.

All of that said, I don't think the standard "whatever works for your team" answer really helps this particular question, since this thread seems to be asking about at what point one's *own* team has crossed a line. That calls for a level of introspection that the other threads lack. I would try to separate any frustration you as a student have from the objective facts of what is going on, and the subjective reality of how inspired these kids are by this program you've got.