Final Fantasy VII Remake puts an incredible twist on the original game, ramping up the action while trying to stay true to the game that set the world on fire in the 90s. In the Mako Reactor mission, you'll attempt to battle your way towards the Mako Reactor to sabotage it. When you finally get there, you'll need to plant a bomb and have the option to set the timer for 20 or 30 minutes.
The important thing to know about this part of the game is that you will have to fight the Scorpion Sentinel after you plant the bomb. The Scorpion Sentinel is a very tanky enemy, and the fight will take some time. The good news is that the timer doesn't start until after the boss fight is over, so there's no need to feel rushed, and you can take your time to avoid making mistakes in the fight.
If you set the timer to 20 minutes, Barrett will tell you that you're arrogant, and if you set the timer to 30 minutes, he'll laugh at you for taking so long. There's no way to win with Barrett at this point in the game, so just let him talk his trash and pick whatever time is right for you.
It really doesn't take long to escape the reactor, and it should only take about five minutes. Twenty minutes should be more than enough time to escape when the boss fight is over.
If you don't get out in time, the explosion will wipe out your team, so be sure to scramble after completing the Scorpion Sentinel.