In the guide, 2.4V is mentioned as the max VRin on air/water.
I really wouldn't be going 1.5V in 24/7 usage. Hell, I wouldn't even be going close to it. Regardless of what other people may have mentioned, I disagree.
This is coming from a person, who took his 3570K till 5.3Ghz @ 1.760V on air (only because I knew it could be done in cold weather). But I just don't have that same feeling for Haswell.