Three Questions.

1. How many weeks until the PS3 reaches 15 million?

2. How many weeks until the 360 reaches 20 million?

3. How many weeks until the Wii reaches 30 million?

^ Please post answers below. 

I will take a punt and say 8 weeks is the answer for all three questions.

PS3 will not get there in eight weeks.

MGS 4 will significantly boost PS3 hardware sales for a few weeks and it could achieve the remaining 1.9 million in 8 weeks time to reach 15 million.
Wii and 360 reaching their milestones is probably given. Wii sells around 375k per week to get the 30 million mark in 8 weeks time. 360 needs to average around 125k per week to reach 20 million mark.

PS3: 10 weeks
360:    7 weeks
Wii:     8 weeks

360: 8 weeks.
PS3 : 10 weeks.
Wii: 9 weeks.

PS3: 11 weeks
360: 8 weeks
Wii: 8 weeks

So I agree with your 8 weeks for 360 and Wii, but I don't think the MGS 4 bump will be that big. Over the course of the next 10 weeks I think PS3 will fall short of selling 190k weekly, but it should get to at least 172k per week so I end up with 11 weeks for PS3.

PS3 = 15 million consoles.
XBox 360 = 20 million consoles.
Wii = 30 million consoles
Those are real milestones not lame milestones like Wii another 0.1% increase in total hardware.

So between 8 to 10 weeks all three of the milestones will be achieved for the three current gen consoles. 

 Well, checking the hardware sales-chart on this site before and after GTA IV launched:

180,000 => 320,000 for the PS3 for a game that's launched on both consoles.  I'm guessing that this real exclusive should perform somewhere in that region.  I can't say whether it'll do better or not, seeing how it's hard to predict the impact this game will have, but a surge of 100,000 hardware units in the first week it is launched, is not beyond impossible.

Seeing that the PS3 has only sold some 150,000 this week, it might take quite a while longer for it to reach 15 million, but I think that MGS4 will lower that number of weeks considerably. 

Wii = 6-7 weeks

360 =8-10 weeks

PS3 =8-11 weeks