1 day!
When I did my 1st initial interview, they called me the next day. Than got to prepare for about a week. The best advice that I can give you is BE YOURSELF, be CONFIDENT, and Tell them about how passionate you truely are about flying. I miss to point this out when I did my interview, plus be Clear on what you want to say. There are loads of stuff in here to help you out in terms of questions they will ask you and so forth. Don't let the nervousness get to you, remember this is your best chance in becoming a pilot! Best of luck to you!