Can I safely wait before starting a mission?

In XCOM: Enemy Unknown there was a small window of time between when a mission was assigned to me and when I could respond to it before it disappeared. This was helpful because sometimes I needed just a little more time before finishing a useful bit of research, and could wait a few hours before heading off to slay the alien menace.

However, when I try to wait in XCOM 2, I get a warning popup that seems to indicate that the mission will be ignored unless I respond to it right now. This happens with guerilla, council, and reprisal missions. Still I find that sometimes I'd like to wait a few hours before dealing with the mission. Am I able to do so safely, or will I always forfeit the mission? If I am able to wait before starting a mission, how long do I have?

Solution 1:

This took a little hunting around to find, but here's a quick breakdown.

Guerrilla Ops, Supply Raids, Landed UFOs, Retaliation Strikes, and Council missions last for a random amount of time, between 18 and 26 hours. This time is determined when the mission is first created and presented to the player. For Guerrilla Ops, where you have 3 missions to choose from, they will all last for the same amount of time.

Other missions, including Alien Facilities and story missions, do not expire.

A "Point of Interest", which is the name for the rewards generated after completing a mission, will last between 9 and 18 days.

Missions will not expire while the Avenger is in the air, but the only effect this has is that you can't expire a mission you're flying to. If you're flying somewhere other than to a mission site, then missions can expire as soon as you land and aren't interacting with some other dialog.

_{Anyone interested can find the majority of the relevant code in X2StrategyElement_DefaultMissionSources.uc and XComGameState_MissionSite.uc.}