How much does it take for the aliens to prepare a dark event in XCOM 2?

Solution 1:

You can't know from looking at the list, it only shows the times until next retaliation and next alien facility. So you can only guess at how long you have.

The usual time to prepare a dark event is between 6 and 10 weeks (on Legend, probably half on lower difficulties). This may depend on the type of event itself. I'm also fairly sure, there is a random factor built in, as I had the same dark event complete after different time spans in my game.

Since new dark events appear only at the start of a month, you can however estimate fairly well how long until the event completes on Legend difficulty. If a dark event is preparing for over 2 months, you can be sure it'll fire very soon, if it's there for a month, it may fire at pretty soon, if it's new, you will have at least a whole month to handle it.