"Most" (from posts in another thread here) note 15 minutes for the interview--mine took less than 10 (and would have less, his computer terminal was "sluggish")--but others have had (for whatever reason) extended interviews lasting over 30 minutes. You are cutting it too close.
Mine at JFK was 5-10 minutes. Asked what my current employment was, what countries I've been to in the past 5 years, took a pict, finger prints, and did a kiosk demo.