From the perspective of a person hiring, I'm sure it's better to hire for exactly as long as you need. The point of 20 minute chunks is to make the idea more appealing to experts (who are busy doing other things). As an expert, you know that if you accept a job, you only need to commit up to 20 minutes, and you'll get paid for a full 20 minutes (even if they're not all used), to justify the 'switching cost'.
