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?