What is left to do once all the puzzles are solved? And when does the game end?

You continue playing the game.

Like BW said, you keep playing.

That's the open-ended nature of the game; it doesn't force you to end just because you finished the puzzles. Some get bored at this point, others like watching their villagers do what they do.

Well, Lurkily, I can tell you what I'd like to be doing with some of my finished tribes right now, but I think you already know. Do I see you working, Lurkily? Are you working, Lurkily? Because I don't see any work being done, Lurkily.

I am, working, Tuna. I'm making eyeballs in photoshop. And right now, I think I'll get some sleep. Long day of real-world work tomorrow.