Until she comes to you in a panic asking for it. Or the meaner route is to go to Target, look at the games, point out one that you know she'd like, say you'll get it for her, and then wait for her to say "uhmmm....I sorta don't know where it is...." before you do the big reveal.

What is your favorite movie ever?
Rock Chalk Baby!! If you aren't from Kansas, you just won't understand!

Dame Kenz Matilda Jayhawk-Rocksalt, heir to the family diamonds.