If you have recently interviewed and have not heard anything, you are probably hoping that this post will tell you a very specific period of time to wait before following up after an interview. Unfortunately, there is only one way to know for certain, and the answer can only come from the potential employer. You have to ask during the interview.
I could tell you “two weeks,” but that could create a problem for two reasons. Firstly, many employers are taking many months to select candidates. If you follow-up after two weeks, they may consider this far too early. “We aren’t even done interviewing candidates,” they may think. Alternately, many companies are expediting the hiring process. Wait two weeks and you may just be too late.
The only way to know when to follow up is to ask during the interview. “What does your process look like?” is a very easy question that should provide you with some understanding about the company’s timeline. It's also a killer question because it shows 1) your interest 2) your respect for process 3) your desire to be prepared (should there be multiple rounds of interviews, etc.).