At what temperature would a hotspot in a cement kiln become visible to the eye?

That depends to some extent on the human eye, which is variable. I can personally see them above 450 degrees C.

What is the optimal forced air flow on the kiln shell? How does this impact on the refractory life? What is the interval in the shell temperature when I should turn on and turn off a fan?

I would say the optimum forced air flow on the kiln shell is zero. The higher the air flow then the higher is the loss of energy through the shell. This means greater heat requirements in the kiln and more penetration of salts into the refractory and therefore a negative impact on refractory life. However, as the refractory lining wears switching a fan onto the shell can be useful to try to build coating. I would suggest switching the fans on at above 350 degrees C.