For depression recovery (which is what my program was focused on), he recommends at least 60 minutes of exercise 7 days per week every single day of your life. In his books Proof Positive and The Lost Art of Thinking, he may make other recommendations for *general* optimal brain performance -- I don't know details on that. While we were at the program, they had us exercising 2-3 hours per day to get us jumpstarted.
He recommends whatever variety of activity will keep you active and interested. Walking is an easy and accessible activity for most people, and the body is well-adapted to it, so he most often advises starting with that. However, for long-term exercise he recommends varying your routine so that you won't get bored.
He does recommend interval training to avoid under-training and over-training, and also to help continuing to build fitness level consistently over time. At the program, he gave us a detailed formula to help us figure our personal intervals, and we also had heart rate monitors so that we could alternate EXACTLY between the top and bottom of our personal interval zone.
The low-tech way to do it is just to work "harder" for a short time and "easier" for a longer recovery period. Here's an article from Mayo clinic on using interval training: http://www.mayoclinic.com/health/int...aining/SM00110