This has bugged me for awhile.

So according to the wiki/the internet, the Mutants start invading the world after a certain point to fulfill the Unity. Starting with Necropolis, followed by the LA Boneyard, then the Brotherhood, Hub, Junktown, and eventually Shady Sands/your Vault.

Thing is, you get some seriously conflicting reports on how this actually works in the fully patched version of the game. Some people say you have exactly 90 days from start to kill the Master if you want to save everything. Others say it doesn't really matter, what matters is that you don't visit the towns after they were supposed to be destroyed. And a few people say the time limit has been completely removed, and the only one that can fall is the Vault after 500 days.

I've now beaten Fallout 1 twice. Once at release, and again 2-3 years ago. Both times I basically rushed for the ending, as I knew the time limit existed and didn't want to test it. So I'd do all the quests for Shady Sands/Junktown/the Hub, but would never join caravans, or really go exploring. And I'm pretty sure I passed that 90 day mark on my second attempt, but I can't remember if the Followers got the good ending or not.

Now I'm playing it again and I want to fully take it in. I want to check the Vault to the east. I want to go fully explore the Glow. I want to do a few caravans for Crimson Caravans. I want to make a whole bunch of caps and buy some cool stuff. I just also want to make sure I know how the mutant attack stuff works, so I can actually plan for it.

Necropolis is the only invasion active in 1.2. The ending slides are bugged (eg you can't get the good Followers ending because the associated quest was removed by Interplay). There are unofficial patches that fix this.