I don't know exactly how far in front it starts, but at my first-lap T1, around the first third of the Mulsanne straight, I've been about 30 seconds behind. On a few good runs I've been 29.mid" behind at that point. After Mulsanne Corner, I'm around 26 seconds behind.
A video might help - 9':06" is good in anybody's book. Don't kill yourself trying to make the video, but thanks in advance if you do.
OK, well, after another hour, I've finally cleared this mission. I would have cleared it after about 30 minutes, except the Sauber swerved into me at Arnage on the last lap and gifted me with a 5-second penalty, which he used to run away. I think I'm going to take a loooong vacation from the prototypes and from Le Sarthe. Unfortunately, my next open mission is at Suzuka, which is my least favorite track in all of GT4.
My final time on M18 was 9':09.low" with a margin of +5.mid" over the Sauber. I'd have been on par for a 9':07" run, but I bobbled the very last chicane on Lap 3 and killed my speed right before the finish line.
Thanks to everybody for your advice and videos!
Wow. No comparison between PAL and NTSC!!! None at all!
I also cheated. I spent hours trying to do it correctly, but I just couldn't seem to put 3 perfect laps together. I was too slow on the series of higher speed bends toward the end, but every time I tried to push on those curves I would go off the track. I just got sick of it. I did exactly what meister_dan did.
Note the extra 15 seconds required to pass NTSC version versus PAL.