Longer lines give the kite a larger wind window allowing it to pass longer through a larger power zone. This means as you fly the kite through the
center of the window, the kite remains powered-up longer before resting at the window's edge.
Longer lines also allow the kite to reach up into cleaner wind. This helps if you are flying in an area with poor ground wind.
But longer lines also slow the kite's control response and require a larger flying field.
My personal rule regarding line length is to fly only as long as necessary in regards to the wind and field conditions. I commonly fly between 25 and
30 meters using longer lines only if I need to reach higher to overcome poor low level conditions.
Info: 30 meter lines is a common industry standard for land and water kite traction.