Wait 30 minutes before making any decision about a candidate's ability to do the work.
First impressions are largely about style.
Style, or the lack of it, has more impact on hiring than substance. The real problem is that once we accept or reject a candidate, the evaluation process shuts down. The quicker this happens, the less new information we seek out and process. We still go through the motions of asking questions, but we either use the answers to support our first impressions or ignore them if they seem to conflict. Emotions play a powerful role in the interviewing process.