They tell you on the order page, but typically within a day or two for non-BTO products, possibly longer if it's BTO. See also http://store.apple.com/us/help/shipping_delivery. You can also check your order status for an estimated shipping date.
Did you order a custom config just to get the 4mb? If so, cancel it, order one with the base amount, and buy your ram elsewhere. I just bought 4GB for my MB from newegg for $79, and it's a 2-minute installation. Not only will your MB ship quicker, you'll save a bundle over what Apple charges for that ram upgrade.