I do four to five sets of 20 seconds, then about one minute of rest. Interesting that our numbers reflect the idea of recovery time being longer for a lengthier exertion. I do hills, too, and my hill sprints are also in the fifteen second range but recovery is more like 45 seconds for me so far, so perhaps I'm just a lightweight after all.
I usually end up three blocks from where I started with each set. It's a little disorienting. My brain has yet to understand that my body can do that. But I've only been doing sprints for a few weeks. Hopefully I'll get up to the seven or eight set league soon enough. And as for the barefoot academy, I just recently enrolled and my feet are still taking prerequisites.