How do I extend the duration of my Private World?

To extend the duration of a Private World, purchase a World Fuel from the online store. You will then be sent a key via email. On receiving this, head to the Shop > Redeem tab in-game and simply input the key to extend the duration of your World by the amount of time you purchased. These are available in various increments.

World Fuels are universal across both types of planet, and all sizes. The amount of time they add scales with the size of the World. For example, a Lesser World Fuel will confer 30 days to a 3 km World, 15 days to a 4.5 km World, and 7.5 days to a 6 km World.