Delivery time depends on when the dealer's next available allocation slot is. You will have to ask your dealer about delivery times.
Each dealer (or maybe dealer group) gets a set number of "allocation slots" from BMW. A slot is a reservation and commitment for a car built during a specific week. Dealers are committed to buy the car for stock if they don't have an customer order for that production slot. Dealers and/or BMW assign the production slots to a dealer well in advance.
The current offer has helped sell Z4s. When we picked up our car last week, our dealer told us his Z4 delivery dates were now 15+ weeks away. But that is just our dealer. If a customer just canceled an order, a dealer could have a free slot available for this week. You will have to ask.