In the picture below you have 48m (2x 24m) Lines with a 17m Kite.
Wind was about 5-7 knots using a hydrofoil.
I tried to get going with normal 24m lines on the same kite on the same day, it wasn't possible.
It's been discussed on other threads before, longer lines give you
LONGER power delivery during the power strokes, so during the water start, for example, you have the same power as normal lines but for a lot longer, which makes a huge difference in marginal winds, especially for heavier guys like me.
Once you are up and planning and generating apparent wind, line length becomes a lot less important and arguably a downside.
We could also argue that with longer lines you catch stronger winds higher up, but that's debatable.
The longer the lines the less maneuverable your kite will be and the more bending effect you will have in your lines.
Another advantage of longer lines is you have more time to correct mistakes and more room to downloop the kite should the kite stall or when you can't steer it back up cause it's too low at the edge of the window.
I'd say for general 12-15 knots light wind riding on twin tips, 27m lines are great and I wouldn't bother going over 30m.
Now for ultra low wind stuff (sub 10 knots) it's great to go above 30m lines, but by then you have a TON of light wind specific skills and you're most likely using a hydrofoil.
I currently use 35m race lines (thinner) with a 12m Kite for sub 10 knots, works a treat, no need for longer. I can get down to roughly 6-7 knots. Thin lines reduce drag and the bending effect and in my experience are true ''game changer'' in ultra low winds, i.e. you're better off getting thinner lines than lengthening normal lines.
The secret is, experiment with line length and find what works best for you! All kiters should do that really.
Christian