What is Terraria?
Terraria is a 2d adventure game wehre you explore around different worlds to collect different loot and fight different bosses. There are many different possibilities for each world and each world is unique.
How long does each world last?
The world will continue to run until everything on the world that you can do has been done. Once that happens, it will be archived and a new world will be started, so that you don't get bored of playing on the same world.