You have to follow up asap. This is also part of the job hunting process. Following up is an important part after the interview. It shows that you are interested in the job and is an opportunity to speed up the process. I know it is very frustrating when you don't get any call from them but unfortunately this is what most employers do after the interview. It has become some kind of a trend lately...
So don't get discouraged and call them the day after the interview. If you were sent there by recruiter, call him/her after the interview.