General concesses is shorter but that doesn't apply to all kites. It also limits your wind range, so either keep some extensions around or have a big quiver
I found 27m gives you time not only wiggle around on the wave but more time to recover a falling kite if you stuff up your timing while using bigger kites.
I found short lines great, but for me, only when the wind was consistent.
It also depends on kite size. On extreme, put a 5m kite on less then 20m lines, and you end up with a bee on acid. Same time use a 12m kite on 25+m lines, and you can have a beer waiting while it turns.