How long to wait after sending an email to a professor before I should assume that I will never receive response? I have sent  an email to a professor at a local university. The email asks for assistance with a fairly large science fair. I have done much of the work and the email was asking about how I could improve it. 
After 4 days I have not received a response yet.

Usually people check their email on a fairly regular basis (Once a day, usually) and so I am naturally worried about it. 

Is it possible the professor has not read the email yet or should I email other professors?
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'.