It is endless. Your main objective is to kill all the bosses, but you can play freely after that. Im currently at day 1400 in my first world and everything works just fine. After you kill all the bosses, the only limit is your imagination :)
You can build, explore, have fun projects with your friends and much more.
Death actually doesnt end the game. It just ends the fun for a while. You will respawn in your base but without any gear you had in your inventory and with 5% of skill points loss. You will still be able to recover it, but it can be a real pain in the a$$ sometimes and can lead to more deaths and loosing more skill points. So try to survive