There is a good deal of misinformation in this thread. The importance of alumni interviews differs by university. Some value them more than others. Some schools have standard questions and those answers, like essays, are read by admissions committees. It’s simply more information for the school. I have done alumni interviewing and the school lets us know the results. I haven’t heard of any schools that interview students that haven’t applied. Mine only interviews students that get by there first hurdle. In some respects, they help a school with yield as they look to determine which candidates are motivated to attend the school as opposed to being accepted. I doubt its a coincidence that the ones rated the most highly gain admission. If you think they are worthless, then just don’t do them. My advice is that if you are chosen to be interviewed, accept the interview and prepare for it.
As far as time admissions committee spends on each applicant, it depends and you can’t take an “average”. Some might only be two minutes because grades and test scores miss the mark. Some schools do more than one round. Admissions processes are different depending on the school and you cannot compare smaller schools to larger state universities who rely more on statistical information.