Just finished my first complete playthrough (not totally done because there are a ton of caches in Skellige, and it is tough selling all the crap you can collect).
I restarted at around "The Bloody Baron" at around 70 hours in, which means this playthrough took around 420 hours, according to the in game counter. (According to Galaxy I'd played for 15 hours... I think it might be a bit confuddled there, it also missed around a third of the achievements, though I don't really care about that... ).
Seldom used fast travel, had a reduced boat speed (halved which improved animation behaviour, if not sailing logic), and explored all the 'marks' on the map, rather than only the map icons... so those rocks/buildings/cliff clusters etc and road ends that all suggested "here be dragons"... Lots of additional mini quests and other interesting bits 'n bobs from just poking about. (including 2 places of power I've not identified on any online guides).