Interview was 30 minutes, tour was an hour. They have the ambassadors chillin waiting for people to get out of interviews and then take groups on tours (like 5-6 people interview every half hour, so you would probably just tour with them). And like FS said, if you want to tour on Saturday, you can just show up anytime and do one. They are really chill about it.
Also, if you're flying out of Manhattan, you would probably be fine just touring Sunday after your interview, because that airport is tiny. If you're doing KCI (or MCI, whatever), then I would probably tour Saturday so you can leave to drive right after your interview (2 hours in fair weather).