I can't say about individual playthroughs. I estimate something around 100-120 hours each. Game Client give it to me in days for each individual run.But the total, clocked by GOG Galaxy is 1.550 hours so far. I'm on a break right now, playing DA:I while I await for B&W expansion. =)
I have over 10 playthroughs with each of the previous Witcher games. Been playing them since 2008. Had a huge anxiety crisis near may 2011 when TW2 was about to be released and then the huge download line for pre-orders pre-downloaded activation. IIRC, CDPR server suffered a lot that day! Gladly they upgraded it for TW3.
I found out about The Witcher browsing Gamespot recent released games reviews, looking for something new to play after Gothic 3, which was a good game but due to dispute between devs and publisher ended up without tech support. The community worked awesomely and released a huge patch that made the game great as it should have been from start.
And TW1, with the Enhanced Edition just released, received a very appealing review, especially compared to the original release which received some criticism because of bugs and some flaws. The effort made to fix it, including re-recording voice overs, was irresistible to me. I love companies that care about their product enough to do whatever needs to be done to make it as perfect as possible. And TW1 became my favorite game then and it is one of my favorite games still.