This achievement is MissableLife is but a dream is completed as the very last thing in the game, during the credits.
When you are about to finish the game
as soon as this happens, stop touching anything at all.
The credits will start rolling and when they end the screen will turn black with sounds of humming and water.
As soon as the credits starts rolling, let it go on for approximately 31 minutes without touching anything to complete the achievement.IMPORTANT NOTE:
If you are using a controller with an auto turn-off feature make sure to turn this feature off, as the game will register an input when the controller turns off and stop the achievement from completing!