How long can I store cooked lentils?
Depending on the temperature in your fridge, cooked lentils will be safe to eat for 2 to 5 days when using a sealed container, filled with the cooking water. By immersing them, your lentils won't dry out and they'll be ready to use whenever you need them - just drain the amount you need. 

If you've got sufficient freezer space you could also freeze the cooked lentils. Just prepare a whole lot of lentils and divide them in individual portions to put in small freezer bags. Make sure you push out any air before sealing the bag. Freezed lentils will be fine anywhere from 4 up to 6 months. 

However, I would strongly recommend using dried lentils, which I'm sure you'll be able to find in Copenhagen. Just make a habit of soaking the amount of lentils you need for your breakfast and you'll never have to worry about storing leftovers. It's a lot cheaper too! But above that, dried lentils tend to taste much better than canned lentils. :)