Ha! Well, to expand a bit on my previous answer,, I probably spent a good hundred hours each on GTA III, Vice City and San Andreas while barely touching the main story of each (although I finally got around to completing the story of III and Vice City). Keep in mind GTA V has more to do (as you said) than any of the previous games ever did, and a much larger map than any of the previous ones.
To be fair to The Witcher 3, it's also probably going to be jammed packed with things to do, along with having branching paths, so if you want to try and get at least two unique playthroughs then you can at least double your playtime. Plus i doubt you're going to be just running from quest to quest with nothing to do in between.
Hm, maybe this is tougher than I thought.
Edit: Geez it took me about ten minutes and five different attempts at posting this reply for it to finally go through.