I'll try to offering short easy answers, first. If somebody shows me why my answers aren't very good, I'll try harder.
I think the easiest answer to the question, "When is it appropriate to challenge the consensus?" is "When experts challenge the consensus."
But when is it appropriate to reject the consensus?
When no one among the consensus is able to provide adequate answers to the challenges.
Let me add one more:
When is it appropriate to accept the consensus?
When all challenges to it have been adequately answered.