start to gather allies for defending Kaer Morhen and assuming you already did their side-quests and just need to talk to them
, then you are pretty close to the end. maybe 15 more hours, maybe less/more depending on you play-style... hard to say it's really hard to get a notion of time with such an absorbing game.
I did a fast (second) playthrough from 'the point of no return' - which should be just a bit further - and that took me pretty much a whole day.
So I guess 12-15 hours of main-story is in the ballpark. When you're playing it for the first time however and do side-quests, enjoy the environment and so on, I have no doubt you can easily get 20hours+ from the game still.