IMHO lines should be just long enough to reach from the straps to the kite.
O.k.,seriously, wanting short lines to set up quickly and fly on a crowded beach, I tried making temporary shorter lines out of a 85ft/26m set by pulling the line through the sleeving,rolling up the spare and taping it up,until I thought I had eliminated most
of it's length.The Gem seemed very
close on these lines and I initially estimated I was down to about 10m or less so was quite surprized to measure I still had around 50ft/15m.
By contrast,joining two 25m sets to make a 50m set was less dramatic.