I was thinking that a couple hours before they open should be alright (in a fairly small town). I read that the people who got there around 8am for the gamestop preorders were able to get theirs. (I think Gamestop's open at 10)
"Down in northern Kentucky, Ben Kuchera was able to land in second place on his local EB Games' list. He arrived a few minutes after 8am and after positioning himself in front of the door, was quickly joined by four other gamers who had been waiting in their cars for a line to form. All four of them were excited more by the prospect of owning a PS3 than by any particular game, and one said his console was intended for eBay instead of his living room."*
Still, I'm going to call a Gamestop near the ToysRUs I live by and see how the atmosphere was for that prerelease, then based on what they say, I'll add two hours just to make sure. I figure more people will have heard about this than the Gamestop one since an ad's going out.
* http://arstechnica.com/news.ars/post/20061010-7951.html
Edit: I forgot that the preorders at gamestop took place on a weekDAY. Also, it seem a lot more people know about this than the Gamestop preorders I might get there 7 or 8 hours early now, but I hope that's pushing it.