To piggyback @antiquity, my good sprint workouts went like this:
10 min dynamic warmup--jog 400m, walking lunges, knee hug stretch, leg swings, hurdle step-overs, and 2-3 build up sprints. I kept moving and went back and forth on the 100m stretch of a track. I'd alternate between a dynamic movement and light jog til I made 3 passes on the track, then did my 3 100m build ups. At this point I'd covered 1000m in about 10-12 min.
Now for the workout: Sprint 100m X 5 with 60 seconds rest. Goal time was 16 sec or less. Rest 5 minutes and repeat. Every week or two I'd drop a second off the goal time. Never went below 14 seconds though.
That type of sprint workout should take you 30 minutes or so but it's structured and the rest is required to perform fast enough to call it sprinting.
My current sprints are typically done indoors now. My gym has a track with 80-90m straightways and about 30m curves. I run fast on the straights (bout 14-16 sec) and walk the curve and repeat. My rest ends up being about 60-80 seconds. I'll do 6-10 fast runs depending on the day. As you can see I'm not in very good shape now compared to my track workouts. That's why I call them fast runs instead of sprints. The indoor running takes about 12 minutes if I do 8 runs and walk 1 lap to cool down (6 laps=1 mile).
Hope that helps you see a couple different sprinting type workouts and their duration. My track sprints were to get me faster; my current stuff is for conditioning.