The optimal duration and frequency of high RPM sprints on a trainer to improve leg speed are largely dependent on individual factors, such as current fitness level and goals. A general guideline is to start with 10-15 second all-out sprints, with 2-3 minutes of active recovery in between. This can be done 3-5 times a week, with a gradual increase in duration and frequency as fitness improves.
Incorporating sub-maximal efforts can be beneficial for building endurance and efficiency at high cadences. This can be achieved by adding longer, less intense intervals (e.g., 30 seconds at 80% max effort) to the workout.
As for translating high RPM sprints to outdoor riding, the benefits lie in increased pedaling efficiency and power output. To reinforce these benefits, outdoor drills such as high-cadence hill repeats or sprints on flat terrain can be effective.
It's worth noting that proper bike fit, gearing, and pedaling technique are also crucial for optimal leg speed and power output.