You can change the time that transitions last, so it doesn't matter how many shades you have that it will be just as long, but think about it like this
Totally White---------------------------------------Totally Black
|---------------|---------------|---------------|---------------|
Start---------------------------------------------------------End
Start/End might be the other way round I forget, but how it is is like
imagine that the time from start to end is however long the transition lasts.
and however close to black or white is where it shows the next map.
I'm not prolly doing a good job at explaining it. Sorry X_x