Honestly, there are no simple answers to your questions. The various ways SHTF can occur means there are just as many things to be on the lookout for.
A stock market crash will occur in one day. That is how it is today with flash crashes.
A hurricane coming gives you warning. An earthquake gives you none (except for aftershocks).
War happens gradually as troops are massed and equipment built up; conversely, nuclear war can happen in a nanosecond.
Personally, from my own experience, whatever the calamity is, when it happens, it will likely happen pretty fast, so you need to be ready.