How long to wait after sending an email to a professor before I should assume that I will never receive response?
It depends on how long you think it takes to answer your email!

If you are just asking a few simple questions (= request for information), you can politely email again the professor after one week. Make sure you make your email as simple to answer as possible (bullet points, yes/no questions).

If you are asking the professor to do something for you that goes beyond answering an email, it is more complicated. An absence of answer can often be interpreted as a 'no'.