After emailing a professor and getting no response from him, how long should I wait before sending a second email? From time to time, I come across this problem: I send an email to a professor asking him about something (e.g., making an appointment to ask his opinion on something) and I do not hear back from the professor. In this case, how long is the reasonable amount of time for me to resend a similar email?
It all depends on the professor, but in general I believe you might get the same result with the second or third email attempt.

I would suggest you try another method to reach the professor. Maybe see if he has allocated to receive students, for questions and guidance. If not, you can try approaching him after a class (be that class yours or not). 
The main point is that you try a physical interaction, after all your professor must have hundreds of students and most with issues and concerns. The email is a fast communication lane, but it is still dependent on a persons availability.

Don't discard going directly to his office and asking if he has the time to see you for a quick question. A professor is there to help you and, I like to believe, they take gratification in doing so.