I've just arrived to Skellige in Witcher 3. How deep am I into the game?
I'm in awe with the game and I play it rarely, bits by bits, so it lasts longer, so I try to count every single chapter.
How deep in am I?
You can go to Skellige at about 5% of completion. The game is set up in a way for you to find Ciri by doing a bunch of different quests around all parts of the game world.
Just because you went to Skellige doesn't mean you have completed anything except the tutorial area and unlocking Novigrad.
There is no way to assume how far you are just by being in Skellige.