While i'm new(ish) to wine making i'm going to guess different amounts of time for different types of yeast used. I recently read the pdf file for Lalvin D47 yeast and it says "When left on lees, ripe, spicy aromas with tropical and citrus notes are developed". Others may not have such a nice imparted flavor i would assume.
Rack out of primary per kit instructions. Leave in secondary at least as long as the instruction say. The lite lees (mostly yeast) in secondary won't hurt your wine.