Every few days in the /r/jobs subreddit, someone will ask “It’s been N days since my interview, and I haven’t heard back. When can I follow up? How long does it usually take?”
Two big lessons here: 1) there is no such thing as “usual” in the job process, and 2) the time to ask about timeframes is before you leave the interview.
Here’s an excerpt from chapter 8 of my book, Land The Tech Job You Love:
[After specifically stating you want the job, ]ask about follow-up. Ask about what the next steps in the process are and when you can expect them to happen. It can be very simple.
You: So, what are our next steps? What timeframe are we looking at?
Interviewer: Well, we’ve got a another week of interviews, and then we look at them as a group, so probably the next two weeks you should hear from us.
You: That sounds fine. If I don’t hear back by the 18th, may I call you? Is this number on your card best?
This part is purely for your benefit, so you may omit it if you don’t really care about waiting. However, if you’re like most people, after a while you’ll wonder “Have they forgotten me? Are they just taking a long time?” There’s no such thing as a “usual” amount of time it takes to hear back, so it’s up to you to ask before you leave. This is also a good time to ask for a business card, if you haven’t already been offered one, to make sure you have all the contact information you need.
Be sure to get a specific day, rather than "a couple of days." As I posted last week, "a couple of days" may mean very different things to you than to the interviewer. Leaving it at "couple of days" is too vague, and leaves you wondering "How many days did he mean?"