I just interviewed someone and literally asked fizzbuzz, and their answer took the better part of an hour. Any tips on cutting an interview short without insulting the interviewee?
Give them X minutes (you don't have to tell them X) and then at X minutes simply say, "OK... that's fine. We don't need to solve it here, but I just wanted to get an idea of your coding process. Before we wrap this up, do you have any questions for me?"
Whenever a candidate can't get it on their own, I give progressively better hints until they come close. This helps you gauge how many hints they need in comparison to other candidates and it moves the process forward.
Then move onto a different topic or skill. No one is good at everything or experienced in all fields. Plenty of candidates will do poorly on one question and then nail the next.