I had this question on my mind always, I think there is no hard rule such. One should walk /run at their comfort level and improve from there. I started ealing at 3.0 and jogging at 5.0. Just completed W4D3 and improved to 3.6 and 5.5 speeds. In between I try 6.0 speed for 30 seconds to 1 minute to see if I am ready for next.