The last next gen console to be released, the Xbox One is going to be on the market 4 days from now. How much time does it need to reach that magical 1 million units in your estimate? Here is some data to help you with your guesstimate:
PS2 - Shipped 1 million consoles in 3 days
WiiU - Sold 400000 consoles in 1 week
Wii - Sold 600 000 console in 8 days
PS3 - Sold 1 million consoles in 2 months
Xbox 360 - Sold 326000 consoles in 5 days
PS4 - Sold over 1 million consoles in 1 day
Things working in the Xbox One's favor are the same things that have helped the PS4 break a record. Namely the internet and the ease/awareness it brings with it. The console will also enjoy something the Xbox 360 didn't have, a succesful predecessor. Then again the closest console that sold for such a price was the PS3, which debuted for 600 and 500 dollars.
Taking everything into consideration i think the console will get there in 1 months time, twice as fast as the similar PS3 did. What do you think?