40 hours a week? I've done it in the past it's not realistic. If you schedule one interview per day, you'll burn out quickly. You need breaks in between to digest the previous interview.
I think 40 hours of interviews is to much, I agree, but I think he meant 40 hours of just looking in general. That's a combination of interviews, in person and phone, and the rest of the time looking at job boards and networking.
Personally, I didn't find that any form of break between interviews made a difference (excepting when I had to fly out of state). I was fine doing 2-3 interviews (in person combined with phone) a day when they came up. It's probably a personal thing.
If you do need a break between interviews, submitting new applications or working on your skills makes for a decent filler. Like a regular paying job, you'll get out of your application efforts what you put into them.
Sure, you can practice and reflect on the previous interview in-between. Your level of concentration is so high during a technical interview, as soon as you disconnect you get a big drop. For an onsite for example, you go to 5-10 45 minutes interviews in a row... you're in until the end, then you crash at night in your bed. No way you can do the same thing the next morning.
5-10 interviews in a row seems ridiculous to me. It may be that I focus on smaller companies, but I've never done more than two interviews for any job, and usually its just a phone screen and an in-person interview.
Then again, maybe you meant that too... :)