Short answer? Entire world population infected/exterminated within 8,640 hours from first xenomorph appearance.
Longer answer? Total extinction of every possible host species within 2 years. A limited nuclear option could not guarantee total containment during the initial stages of an outbreak. Within days of first contact there would be dozens of aliens, or more, especially if egg morphing was used. Escape would be imminent. As the possibility of Alien victory becomes more plausible, the important people down in the bunkers would level us all. The world would be reduced to ashes, as the sun is blocked out and a global nuclear winter sets in. Thus making earth, Acheron 2.0 The last human beings (those aboard the I.S.S. or some similar orbital sanctuary) would pass away due to lack of supplies, or any number of mechanical failures and/or acts of God. So basically, unless you could pull a "looper" and be there at the exact second the first alien joined our world, we'd be completely and thoroughly screwed. I'd love to see it as a movie though. Like WWZ but with Aliens.
