In my opinion, the length isn't a worry so long as the player can walk away feeling satisfied or had fun playing it. Some games go on forever, becoming a test for how long you survive. Some games have a large amount of levels, but depending on how interesting the game mechanics are this could backfire if it's the same thing over and over again. Focus first on quality and keeping things fresh for the player.
If you feel satisfied as the designer behind the game and feel like you've explored all the game concepts that you wanted to, then it's done. If you feel like more could be done and more challenges could be made, then it's not. Push yourself to make a good game, and everything else is secondary.