I’ve been lucky, and have never gone through any of these marathon interview sessions. I think I’d probably just walk out. Honestly, I’ll never apply for a job at Google or Amazon or Bloomberg, based on what I’ve heard about their interview processes. It just seems like there’s a fundamental asymmetry between how interviewers value their time, and how they value the time of others. I think it’s somewhat immoral to make 20 people submit to 10-hour interviews when you know you’re only going to hire one of them.
Several of the responses I’ve read to this post are also troubling. You tell someone you can’t spare more than 10 minutes to read about them, but your company wants to consume 15 hours of their attention? Employment is supposed to be a symbiotic relationship.
Where I work, we have two rounds of interviews, the second of which is a pair programming exercise. The two combined have never gone over four hours. Honestly, if you need 12 hours of watching someone whiteboard stuff out before you can tell if they’re any good, you’re probably not very good at evaluating talent.
All that said, if you’re having that much trouble getting a job in this industry, it might be more than a technical skills thing. Programming chops matter, but your attitude matters just as much, if not more.