Judging from my Elder Scrolls Online experience, it takes me about 175-200 days played to burn out on something I'm really into. But nothing else has really come close to that since I quit about a year ago. I've been kind of bouncing from game to game, got a couple hundred hours into Guild Wars 2, a hundred hours or so each on Grim Dawn and Diablo 3, a little less than a hundred on Vermintide 2. Basically, when I find a game that scratches an itch, I will play it until I've squeezed every drop of content from the bottle. And it looks like I'll give a new game about a hundred hours to hook me, if I can get past the first hour or so.
I kind of want to go back to ESO, but it's 75% less fun for me without the subscription (the Craft Bag is BAE, and the DLCs are where I'd be spending my time), but I'm hesitating on the question of "Is it responsible of me to be spending $15/mo on a game I've already quit playing once?" And yet, nothing else has come close to hitting that mark for me. GW2 is really amazing, it's got a lot of things I'd definitely miss if I went back to ESO, but... it's not the same, somehow.