Till what point do you do exercises while studying? I will try to make this as less subjective:
Till what point do you continue doing exercises at end of the chapter to learn something:
I do it in Grad school and that's where I really learn. Do professors/post docs learning about new fields do it too?
Learning is a life long activity. Graduating with a PhD is not a license to stop learning, it's a license to (eventually) get paid to do research and learn more :)