I'd go with 15 seconds due to what's explained in Q&D page 26-33.
Agreed, 15-16 seconds, like the big cat.
Yes I think 15 seconds would work better.
15 Seconds Sprints
Everyone is on board with 15 seconds sprints; which makes sense.
The underlying reason is that ATP (Adenosine Triphosphate) is the what fuels Power and Maximum Strength.
ATP is expended in around 15 seconds or less. After 15 seconds, power drops like a rock.
Fast Twitch Type IIa and "Super" Fast Type IIb/x Muscle Fiber utilize ATP. Once ATP is depleted, the Muscle Fiber down shifts as follows...
"Super" Fast Type IIb/x > Fast Type IIa > Slow Twitch Type I Muscle Fiber
Thus, if the intent is on training/developing the the "Super" Fast Type IIb/x and Fast Type IIa Muscle Fiber, sprint time needs to be around the 15 second range of less.
Continuing past 15 seconds elicit a different training effect; which is fine if that is your objective.
SMIT, Supra Maximal Interval Training
SMIT require longer rest periods between sprints, 2 - 3 minutes.
That allows for greater ATP restoration to the muscles, which ensure you are able to produce/develop more power.force.
Research shows the following...
1) 30 seconds of rest restores around 50% of ATP.
2) 45 seconds restores (Dr Greg Haff) around 79% of ATP.
3) 3 minutes or longer restores 100% of ATP.
Thus, the longer the rest between sprints, the greater your power/.force production with the following sprint/set.
Tabata Protocol
This involves short rest between sets of 10 seconds.
This method falls into Metabolic Training; great for increasing your metabolism and conditioning.
However, since it limits restoration Muscle ATP Restoration, development of power is limited.