Idle Games at CoolmathGames.com
It really varies from game to game. Many idle games just have players going from stage to stage, slowly leveling up. For example, Tiny Fishing has players progressively catching more and more fish that increase in value. There isn’t really a defined ending, but it will take you a long time to get to a point where progression gets very slow and boring.
On the other hand, there are some games on the playlist that have very defined endings. For example, the goal of Learn to Fly is to take out the iceberg that is several thousand yards away. Once you have enough upgrades to make it to the end and destroy the iceberg, that is pretty much it. While players have the option to continue through the game and see how far they can make it, most just end there and go on to find another game.
Both styles can be extremely fun, it just kind of depends on which kind of gamer you are. Some people prefer a clear and defined ending, while others are just playing idle games to get upgrades and tons of points. There is no wrong answer.