It's important to first explain how a leap works to answer this question properly. Each leap consists of two phases: the fussy phase and the skills phase.
The fussy phase (the yellow (in app) or gray (in book) bar in the leap chart) is when brain development takes place. Because your baby looks at the world differently during this brain development, it seems like your baby has woken up on a different world. And they get upset by that. However, when they have gotten used to this new world, they will show new skills that are part of brain development: the skills phase (the green (app) or white (book) bar in the leap chart).
When a baby shows these skills, is very different for each child. This also depends on the interests of your child. One child will show motor developments earlier and speech developments later. The other child will do exactly the opposite. Your child ultimately determines when they show what. The nice thing is that you can recognize the character of your child at an early age :)
Some skills of, for example, leap 3 your child will show in leap 3, but there will also be skills that belong to leap 3 that your child will show in leap 4.
Therefore it is not important to know how long a leap lasts, but it is especially important when a leap, and with that the brain development, starts.