A really, really long time. They have not offered this option yet with the DX200 so I highly doubt they will with the DX150 either. These units are packaged, sealed and prepared for fulfillment sitting in a warehouse somewhere. It's not as though when you order they assemble and package your specific unit and send it out.
If you're hoping to somehow avoid the cost of buying an amp you will not use, I suggest trying to find a deal on a used DX150 at some point. As others have said, Amp 6 is quite good as far as the "factory default" amp goes and unless it does not provide a feature that you need, you may be happy with it.