Remember, Pedro asks which line length that makes sense for "extreme light wind", and not in general.
And while I agree that shorter lines on the really big ones makes sense if a tad more wind (only when foilkites), I disagree that you dont gain anything in extreme marginal wind with longer lines, on bigger foilkites.
Many who answers here ride in "race" mode so to speak, meaning, if not racing, using short or really short lines as it is more than sufficient except for the extreme light wind.
The latter would be something around 4-5 max 6 knots.
And while used to riding with a certain line lenght most of the time, the advantage using longer lines diminishes rapidly, and you get the "bad things" right in your face as not on your backbone now.
Knowing your gear is by far the most important as we all know.
When on the edge just not able to ride, my friends racing say - they KNOW they could go and ride with longer lines, but they are so used to the shorter ones that they often dont bother switching to longer.
Also because they dont wanna "just ride" - but mostly practice racing.
Longer lines helps, how long till it stops, I dont know, 30 and 32 m is what I have been using for above winds with 12 and 15 m2 race foilkites, maybe at some point you wont gain anything, I dont know where it stops ?
But it is still "pleasant" and easy at 30 m thats why I wont go longer.
If we talk normal wind gradient of course, important to say.
Anyone who has tried even longer, in different conditions - and got experience on this ?
I have an idea, but dont know, that 40 m wont really help much (never tried it).
Might start to feel really sloggy and impractical yes, but it might also be able to get you down even further towards the utmost marginal wind where you can start and ride - so interesting about anyone who tried.
Peter