The theory: After you get a certain number of games, they basically become background noise. Your spouse knows you have a lot, but they lose track and even lose interest in how many. As long as you keep some semblance of order, they won't notice another machine sneaking into the lineup.
I added The Getaway to my basement lineup this afternoon. My wife knows I have this game, it has been in the garage for a week. So I'm not trying to get away with anything. I wasn't planning on putting it in the basement, but a few days ago I took measurements and realized it would fit. So I decided to add it and see how long it takes my wife to notice.
We will be having a New Year's party here, so she's bound to find out by the 31st. So I'm not give a poll choice greater than over two weeks. If it weren't for New Year's, I'd give the option of one month.
An idea of what it looked like before (old photo):
How it looks now: