I'm not sure if my sprint structure would apply to actual running sprints as I do mine on a stationary bike, but I typically perform 8 to 10 reps with 30 second sprints and 60 second rest periods.
I just started this structure about a month ago, and my goal is to increase my output at each sprint session. Currently I'm accomplishing that by adding an additional sprint each week. Once I reach a total sprint time of 20 minutes, I plan on increasing output by either lessening my rest periods or lengthening my sprint periods. Hope this helps!