I had an interview and have not heard anything. I am sure the hiring manager is just really busy as it seems a busy firm. I thought I might call in myself and just ask what is going on. How long should I wait?
Ideally at the end of any interview you should ask when they expect to make their decision - don't be afraid of pushing them for a timescale, you might have other jobs to think about. This will give you an idea as to when to start chasing.
Obviously in this case you have already had the interview so the next step is to follow it up with a "It was nice to meet you, I'm interested in the role..." e-mail/letter within a few days just to keep you at the forefront of their mind. If, after this, you haven't heard anything for 2-3 weeks you should phone or e-mail them to ask if they have made a decision and if you were not chosen then ask them to provide feedback.
Companies should always let you know but not everyone does (sometimes they hang on in case the other person doesn't accept) so you should chase them up for some feedback which might help you for future interviews.
Did you send them a thank you note? That should have been done right away. But if you've not done so, this might be a good time to remind them of you and ask for the job.
A follow up after a week is appropriate but don't be a pest. Some companies don't always notify the candidates they do not choose. It's a good idea at an interview to ask their timeline so you know what to expect.