My sprint sessions usually involve sprinting as hard as I can for as long as I can. There's a point where you feel your stride break, and you know that you don't have any more to give. That's when I slow down and stop. It usually takes 20-30 seconds for the first one or two, and I'll cover somewhere around 150-200m. I rest until I feel like I can do it again. The later ones might only be 15 seconds, or less than 10 seconds by the time I get to sprint #6. The last ones might be 50m, but that's all I've got to give at that point.