XCOM 2 is known for terrorizing the players with turn-limited missions, and this is not limited to the battles. The war against the aliens is also "time-limited" because the Avatar Project will not stop its progression unless the player takes steps in stopping it.
This is sort of cheating, because clearly the game was designed with the looming completion of the Avatar Project, but whatever. No one is judging, and more importantly, no one is watching.
.ini file to tweak: DefaultGAmeData.ini
Change the AlienHeadquarters_DoomMaxValue[0] to whatever you like. There are variations of this line due to whatever difficulty you are playing. So here they are,
AlienHeadquarters_DoomStartValue[0]=0 ;Easy
AlienHeadquarters_DoomMaxValue[0]= [insert value here]
AlienHeadquarters_DoomStartValue[1]=0 ;Normal
AlienHeadquarters_DoomMaxValue[1]= [insert value here]
AlienHeadquarters_DoomStartValue[2]=0 ;Classic
AlienHeadquarters_DoomMaxValue[2]= [insert value here]
AlienHeadquarters_DoomStartValue[3]=0 ;Impossible
AlienHeadquarters_DoomMaxValue[3]= [insert value here]