An interview should be as long as it takes to determine the nature of the person you are interviewing. It could be as short as 20 minutes if you feel no connection, but it could last well over an hour if there is a good give and take.
Hiring managers make a huge mistake by not allowing enough time to get to know the person they are seeing. When that happens, candidates, who are rarely in control of the interview, don’t learn enough about the company and people they meet in order to make a proper career decision. And sometimes they actually get offered jobs despite the short interview.
Some time ago I had a senior candidate (general manager) on an interview. I was a little surprised when the firm he was interviewing with scheduled six interviews at half hour interviews over three hours. Aside from being exhausting for the candidate, it is unfair to both parties. The individuals he met couldn’t possibly have found out enough about him to know if they liked him or not.
The purpose of an interview is for both people to get to know each other. The interviewer needs to learn about the qualities of a candidate that would