A Googler writes on a blackboard at the company's Berlin office. Adam Berry/Getty As a hiring manager, if you research a job candidate and ask the right questions, there's no reason the interview should last longer than 30 minutes, Google chairman Eric Schmidt and former VP of product Jonathan Rosenberg write in their book "How Google Works."

"The shorter interview time forces a conversation that's more protein and less fat," they write. "There's no time for small talk or meaningless questions. It forces people, including (especially!) you, into a substantive discussion."

While it's possible a candidate might have several interviews with multiple Googlers, typically each one won't last longer than 30 minutes.

Google trusts its interviewers to go with their first impressions and determine by the end of the half hour whether they want to schedule another interview. They also limit themselves to a maximum of five interviews, regardless of the importance of the position.

It wasn't always that way.

"One time, in our early days at Google, we interviewed a particular candidate over 30 times, and we still couldn't decide if we wanted to hire him," the authors write.

Schmidt and his team decided that they were going to track interviews to maximize efficiency.

They found that after one interview, interviewers were ready to make a decision about 75% of the time. Decision-making ability gradually rose to 85% after four interviews and then plateaued. They decided to round up to limiting interviews to five, because computer scientists appreciate that five is a prime number, Schmidt and Rosenberg joke.

"Remember: From the interviewer's standpoint, the goal of the interview is to form an opinion. A strong opinion. A yes or no," they write.