According to info in the More Witcher 3 Details thread it´s 50 hours for main quest and 50 hours for side quests.I am a little dissapointed that Witcha 3 is expected to be a 50 hour rpg though. I read somewhere that it's going to be 20% larger than Skyrim, so 50 hours just doesn't seem right.
Besides the fact that these estimations are always nebulous, one thing to consider is that in Skyrim a huge portion of playtime consists of dungeoneering, while talking to characters and engaging in overland questing takes relatively smaller portion of play time when compared to games like Gothics or previous Witchers.
Considering in The Witcher 3 the narrative aspect is supposed not to be side lined by the game´s open world-ness, I think it´s quite reasonable to expect dungeon crawling won´t be as prominent as in Skyrim.
In that light I´d say estimated 100 hours of quest content is more than enough and lines up well with the games like Fallout: New Vegas or Gothic 3.
Anyway, just speculating .