If it's longer then 20 hours. Games that last only a few hours are not a problem as long as they are priced accordingly.

1000 hours

^ That's exactly what I would say. Thankfully most of the games I played had enough content to make them feel worth the money spent. Except Epic Mickey 2, not only was it bad in many ways, it was over in like 5-6 hours iirc.

It helps a lot when you can play quite different character, both mechanic wise (like say mage instead of a fighter) and moral wise - I love RPGs that I can start again right after I've finished them and make a build that I don't usually play and then be on the dark side on top of it.

While I fairly liked Witcher 3, I don't think I have much incentive to replay it, and why I wish CDPR to make more of a 'unnamed hero' type of D&D-ish game sometime in the future.

Yeah, I played through DA: Origins 4 times with different characters and parties. The different interactions between the characters made it worth it, plus many quests had 3 very different outcomes. The Witcher 2 was better too with a completely different chapter 2 and again more splits in the narrative paths in the final act. I also completed that multiple times.

Now I fell into the Fallout 4 trap. Although I'm really enjoying it now, I'm pretty sure it will end in a fizzle and I'll never touch it again.
It was the same with Skyrim and Dark souls last gen. Both great, spend about the same amount of time on them yet Skyrim ended in forcing myself to complete the story (after doing everything else) while in Dark souls I went onto ng+ and ng++. Imo Dark souls >>> Skyrim. The problem with most open world games is that they have a beginning, a middle, more middle, even more middle and nm forgot what why this was fun, feels more like a second job now.

Anything above 40 hours becomes annoying unless I really like the game.

I had no issue with the orders length, it was a poor game regardless which was it's major problem. 5 hours and half was barely interactive, the rest was borderline shooting gallery with scripted enemies.

On the other end we have mgsv which was a lot of fun, but then dragged on waaay to long. And I'm not just talking the optional stuff that many long games have that you can choose to do, or not do, once you've finished the story. At 43 hours it's repetitive and I still haven't finished the story, that's also a problem.