Some people will sink 15 hours a week into an MMO or other addictive game, and that's not even the extreme case (that's like 50h). It's hard to make that actually fun, so just focus on keeping the player coming back. If there's a genuinely fun part of the game, make sure it's slightly more fun when you grind for hours first. Ensure there's some kind of never-ending progress, often in the form of rare collectibles that are added over time to ensure nobody "beats" the game even if they've done everything that's fun in it. Along with customizations, this will make players feel like they lose something if they quit. Also make it seem like they're doing something productive with their time. Some games have a "daily tasks" checklist that makes it feel like a job. Or the simple version is a button you press once a day for free loot, just to get people to log in.

And most video games aren't like this. You'll see a contrast in how addictive games are monetized compared to others. The tactics have been well-known for a while (World of Warcraft pioneered many of them in 2004-06), and in the past decade it's become a lot more common. GTA Online isn't totally an addictive game, but it seems to have a bit of that with the ingame assets.