Yes, there should be some kind of GRACE period after a show has been off the air (initial broadcast run that is, not talking about syndication) before it's released on DVD.
Feh.... you kiddin? They make the DVD's WHILE the shows are being made (same thing with movies.... It's like a theatrical run is the commercial for the DVD's!)
And Pooh is partially correct about the sales of the DVD's bringing the show back. Though, a lot of it also had to do with them buying American Dad and saying "Hey, while your at it, wanna make more Family Guy?" But that's neither here nor there.
I think a TV show should come out on DVD a YEAR after the season finale... but since so many people wind up missing the episodes when they premire, and certain shows no longer doing reruns (Family Guy is CLEARLY not one of those. If I see that episode where Peter is an illegal immagrant again, I'll go crazy. man was that a lame episode) the DVD's come out just before the season premire. The only exception is the Simpsons, since they're basically releasing them year by year, and aren't even at season 11 yet... (or are they?)
Now the Simpsons... that's gonna be an EXPENSIVE complete series set... ooooh yeah.