There is no predictable time frame for wait list notifications. It all depends on the price and ES of the piece, how quickly you got on the list (as bagelsncheesey said), and how long it takes for someone to decide to cancel. Some may come as soon as someone has an attack of buyer's remorse (which sometimes happens within a few days after ordering). Others may come after there has been some lengthy discussion about the piece, causing some one to rethink their decision to order. And still others will come as the shipment date approaches, either due to flex-pay problems, other credit card declines, or people who panic when the 20-day notice arrives, and realize they can't afford the piece at that time.
I've gotten on the wait list for some items within a few seconds of the sellout (as soon as the wait list option appeared on the order page), and still didn't convert until just before the item shipped (and some never at all). For others, I've gotten on the wait list weeks after the sellout, and gotten a conversion notice within a couple days.