On average, perhaps 40 minutes.
But some take longer if you have multiple interviewers or even a group job interview.
A 'normal' interview will include:
- A welcome and some rapport building conversation -- up to five minutes
- Finding out about you, the interviewee, though your answers to interview questions -- about 30 minutes
- Answering any interview questions you have and telling you more about the job -- 10 to 15 minutes