What if you built a whole game around a Tamagotchi, except instead of choosing to feed and care for it, you can choose to interact with the world through it - or not? From the moment you start The Longing and start as a Shade, 400 days tick down, even when you've exited the game. Don't worry, though, you don't have to actually play the game if you don't want to. While your Shade may be lonely, you're free to ignore it or send it through caves to collect items for your underground living room. It's a little bit on the slow side though, so you might instead prefer to read some classic literature such as Nietzsche or Moby Dick, or throw your king's commands unto the wind and explore the outer regions of the cave.
As soon as you start, the game inevitably counts down the 400 days - even when you stop playing and exit the game.
It is now up to you to decide what to do with your solitary existence beneath the soil. Don't stress yourself, you have plenty of time.