When to release a game demo?

Ok, im not close to demo'ing anythng of mine yet, but i'm curious about how
the industry views game demo release dates.

It seems to me that maybe 5 years ago, pc demos were consistantly released
within +/- 5 days of the actual release of the retail game.  But lately, ive
noticed some demo's appearing months before the game is released.  One game
that im currently reminded of is "Silent Storm."  The demo was released a
few months ago but the full game is still not available in stores (actually,
I "think" maybe a European release has just recently been made?????)  After
I played the demo, i litterally would've bought the game that very day, but
now i'm growing perhaps too warm to it.  Is it a mistake to release a demo
so far in front of the retail release?

-PizaZ