In Berlin, depending on which Finanzamt is responsible for you, it can take 4–6 weeks to get the Steuernummer. We hear occasional stories of someone who got it on the spot or in just a week — if it’s very urgent, you can always try dropping the form off at the Finanzamt in person and nicely asking for them help you get it asap, but that may or may not work. : ) Good luck!