When you think that people would be interested in your game. Generally speaking that means that the game looks good and isn't just a prototype made with unity boxes. It also has to have something cool or unique about it. For example if it is a horror game it has to have some sort of unique mechanics, because there are already so many horror games out there.
And another thing. If you want your game to succeed on launch, you have to start marketing at the very beginning. If you start promoting your game on launch it's not enough because you need a steady flow of people buying your game to make the success sustainable. You have to build a community for the game trough posts, memes, ads, devlogs and maybe offering some sorts of rewards for playtesters.