Just make sure to be out after curfew. Curfew starts at 11 PM and you pass out at 2 AM. This is 3 hours. Repeat again the next night for the achievement.
You will know you are past curfew because the clock in the upper left of the screen will turn blue. As you go through the course of the game, you will spend much more than 5 hours out, just completing your missions.