From Apple.com, the day it's released. In stores, it will take a while for old inventory to deplete. In the past, Apple dropped in DVDs with the new OS, now I guess you'll get a Mac App Store code.
My point to the OP was after the release, Apple will have Lion on every Mac it sells. I am a little confused by what the OP is asking. Apple won't install a previous version of a OS on a system it sells. It will always have the current copy at the time of manufacture, minus any that have already been produced.