Given previous statements and incidentally collected data, I believe it's anywhere from 10 to 18 months, depending on the size of the project. Mearls mentioned one developer who had been working on D&D Essentials since September of 2009, as one recent example. In other statements, Dave Noonan has said before that most design work begins about a year out, and development work a few months after that.
I'm betting they're working on products for Q3 2011 as we speak.