This whole "End of World" thing looks bugged... it's 3:34 and i can still exist :/
The world will only end if you make NCR unguarded! Do it! Do it quick!
You know, you can do it every time you want. All you need is ammo, will and time. Reinforcements will end after few waves, so you only need to worry about respawning guards, what may be bit challenging but not impossible. Just ask some oldtimers from BBS, not sure if only, but (i think) first gang which taken over Shady Sands after reinforcements was added. And it was "given back" when they ran out of ammo, so make sure to take few extra bullets.
No point in that since the guards respawn every 5 minutes.
It's 10 minutes