I can't really say how much of the map I've explored. I literally watched and read
nothing about this game before it released because I did not want to spoil it for myself. I can just say it's bigger than I expected and I would be surprised if I've explored even 25% of the map. For example...
[Show spoiler]...in my last playthrough I defeated the first major boss, Godrick. After defeating him I continued forward thinking that maybe I'd find an item or NPC. I kept thinking I need to get back to the map and explore all those areas I saw but didn't check out. Well, instead of finding an item or NPC I walked out of a structure and into a massive new area that looks completely insane. So now I have this massive new area and there are still tons of places I've already seen but haven't come close to fully exploring. Even Godrick's castle (which is basically a massive level unto itself) still has places I never accessed. So much to do!