I think 2 hours would be sufficient for 90% of the candidates. That's acceptable when you're in a start-up environment (where you generally have to fight for candidates and the pool is self-selecting to start with), but the 10% at Google could really make a big difference: google's pool of applications is bigger and the "laws of large numbers" come into play.

Also there's something else I haven't thought which is also a matter of "laws of large numbers": you simply can't hire every qualified one (and there would be opportunity costs in hiring the first person to pass the minimal qualifications).