Final Fantasy 7 Remake Set Timer is one of the few decisions you can make early that will affect if you start off with extra items. FF7 Remake's choices are mostly based on dialogue choices, and they affect your relationships with your teammates.
There are a few choices that alter the game, but not many. As for the Bomb timer, I'll explain the differences.
Final Fantasy 7 Remake is cool...
That is all:
— Daithi De Nogla (@DaithiDeNogla) April 10, 2020
Final Fantasy 7 Remake Set Timer: Should You Set it for 20 or 30 Minutes?
You'll have two outcomes depending on your decision. If you select 20 minutes, Jessie will give you an Ether and two Hi-Potions near the start of Chapter 2. If you select 30 minutes, you get nothing. If you unlock Hard mode by completing the game, you'll only have the 20 minute option and you won't even get the rewards.
If you toward the beginning of the game looking to leave the Sector 7 Slums, you'll need to complete some side quests. Here's our guide on finding the lost kittens.