Actually, I ran some tests, and phase lane length either doesn't affect transit time at all, or it is aboslutely minimal.
However, it does affect the maximum length the phase lanes can be. I noticed changing this setting will greatly affect how many planets are connected. Especially if the system doesn't have a huge amount of planets. (And yes, I am talking about "Length", not "chance")
This is completely contradictory to what the manual says, but its what I've observed.