Average is three months. I ordered an X5 in July and got it in September. My '07 M3 was ordered at the end of August and took delivery in November. It's safe to say three months unless you change your mind and change the specs. Also a 10% deposit when placing an order, and another 10% when the production is confirmed.
2007 E92 6MT - Silverstone II with /// Performance Exhaust with ACM Test Pipes and BPM Sport Stage II Dev-Tune
2011 F25 X3 3.0d