ELI5: What exactly do people do with long term holds? When are you "supposed" to sell?

Hi, I know this post is going to sound like I'm patronizing investors but this isn't the case. I'm 30. Most of my savings are in funds, and more recently in individual stock, and now I'm wondering how long I'm "supposed" to hold (assuming I don't need that money). I am just not sure what happens next after holding I guess. People always say to long term hold certain stock, but when do you actually sell? In retirement? When you need money? Like yeah sure sell whenever you want or need money, but what's the ideal timeline? What is everyone holding for? Just for the value to go up? And then what? What is your plan? Thanks everyone.