Here are a few dates from the build process of my car if it will help out any.
8/7/08 - individual color approved/allocation granted
9/24/08 - production started
10/6/08 - production completed
10/30/08 - arrived at port (Charleston, SC)
11/21/08 - picked car up at Performance Center
I would point out a few things that may account for some differences between the length of time in the U.S. (as Lemans has said) and what BMRLVR experienced in Canada. Canada has officially implemented the Individual program for M3s; the U.S. has not. BMRLVR ordered a "standard" M3 Individual color (Moonstone). Special order/request M3 Individual colors (e.g., LSB, Dakar Yellow, Brilliant White, Monte Carlo Blue, etc) may take longer. In my particular case, my car was approved in August. I understand the factories are shut down in August. So, that may have added some time.
Also in the U.S., there will be some sort of additional time required upfront to get the necessary approvals from BMWNA, BMW AG, etc to order your desired color. The amount of time for that process can be highly inconsistent depending on color, your dealer's knowledge of the process, and who they work with at BMW.