I have played around a lot with mash time to try to find out the minimum time I could get away with and still make awesome beer. The short answer? 40 minutes. The problem is not conversion -- the bulk of conversion is done in the first few minutes. But what is greatly impacted is the attenuability of the wort. You can mash a beer for as little as 15 or 20 minutes, and you can still get decent mash efficiency, but your 1.060 beer might finish fermenting as high as frickin' 1.025 and just stop right there, and no amount of rousing or pitching more yeast will help. A 30-minute mash time is better, and will attenuate okay about 50% of the time, in my experience. But 40 minutes, well then it attenuates just fine every time, and you just saved yourself 20 minutes of waiting time per brew day. If you do wait for a full 60 minutes, you'll only get another gravity point or two of attenuation, so I think the 40-minute mash time is the better way to go, being that it is a great time-saver, with no significant drawbacks. So that's how I've been making most of my beers, unless I want to eek out every point of attenuation possible, which for some styles is appropriate as well. So, it all depends on what you want out of your beer. But attenuation is the effect that you need to look at, not conversion.