As a panelist for technical interviews, you often come across candidates who have all the requried educational qualifications, skill sets and experience level on resumes, but struggle to answer even the most basic questions. Ideally, technical interviews should try to check different aspects of a candidate and test them on various skills. So, if the candidate falters on one aspect, one should test the other ones before coming to a conclusion. But often, if a candidates falters on the first few questions, the red flag rises up pretty quickly. What in your opinion should be the bare minimum time spent with a candidate before making a fair accessment of his/her skills and suitability for the job?
What should be a fair amount of time in an interview before rejecting a candidate?
interview
In my experience, 20 minutes is enough for them to put their foot in their mouth.  I've been on both sides of this.The good thing about dragging it out with a few more questions than you feel are necessary, is that they will know they aren't appropriate for the position.For example, if you say implement an example of polymorphism on the white board, and they say ummm...Polly who?, they won't go away saying I wasn't hired because I'm [insert race/age/sex/etc here].  They will know they are unqualified.  It also helps them know what their deficiencies are, so they can further their own careers (probably not your goal, but it is a side effect I thought I should mention).