I picked the more realistic (in my opinion) 1 week. It took The Sims 2 a week to break that barrier, and The Sims is the best-selling game of all time.
However, what we Halo fans have over the Sims fans is a ferocity when it comes to our game(s). Most of us know when the release date is and will be in stores that day or have already pre-ordered ours. So basically what I'm trying to say is "Gee, I don't really know". What a meaningful post, huh?
*Just a note, I'll be there with the rest of you on the first day trying to pickup my pre-order.
Edit: I'm not positive on the "best selling..." moniker given to The Sims, but I know I've read it somewhere, so don't quote me on that.