I just bought that super expensive Kerastase Masquintense for Fine Hair after succumbing to the rave reviews on MakeupAlley.
How long do you usually apply it? I’ve heard you can do it for 5 minutes, or leave it on overnight. Does the length of time matter?

if your hair is very fine I would not suggest overnight, I would think 15 minutes will do!!
by the way great product to have!!