A gradual increase in sprint duration may be one way to get more sprinting into a workout. Using a pyramid format, starting with an increased duration to the peak and then decreasing duration, can be a progressive way to increase intensity to your workout.  I find varying the duration of your sprint in this way leads you to challenge yourself more. Let's take a look at what I mean...

Duration = 7.5 minutes

Cadence = 100+ rpm for sprints; 80-90 rpm for recovery

Resistance = set tension slightly higher than your steady state, moderate pace (e.g., 4-5 out of 10); during recovery resistance can be turned down slightly to make it easier (for beginner or less fit cycle participants); to challenge more advanced cycle participants keep the resistance the same for sprint and recovery.
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The pyramid above may help visualize the drill. Now take yourself to the peak and back!