I had just a short time to do a bike workout today and headed to the fitness center in the afternoon. I did my first 10-20-30 type of workout. For minutes, I did 30 seconds at less than 100 watts, 20 seconds at around 150 watts and 10 seconds at an unknown wattage. The gears for the respective intervals were 5, 10 and 20. When I was in gear 20 the speed read in the range of 25 mph.
I think that 15 minutes was a good start for this workout. I think 30 minutes would have been too much.