Contacting professors about research: if they don't respond to my emails, how long should I wait to follow up?
I agree with badroit's answer. I would also like to add the following:

I don't know what was in your original email, so this may not apply. 

However, if your email requires some thought or effort on the part of the professor to respond to, then it will sit in his inbox until he has a chance to sit down and compose a proper response. Since you are an unknown student and he is very busy, this is low on his list of priorities and he may never get to it.

On the other hand, if you politely follow up a week or two later with an email in which you ask to meet:


  Dear Professor Y,
  
  I am going to be a student in your department next semester. 
  
  I have been reading about your paper on Y and I am very interested in talking to you about it; do you think we could schedule a time to meet next week?


This email is easy and quick to respond to ("Yes, how about Tuesday at 10?"), so you may be more likely to get a response.

(I know this doesn't make sense, since the professor would still have to expend time and effort to meet with you. But spending a half hour on a meeting on some future date seems much less of a burden than spending ten minutes right now to send an email to an unknown student.)