SiameseMoose, thanks for sharing your experience. While I’ve never actually done a one-after-the-other experiment with the same recipe for two different mash times as you did, I did make 6 small batches of different beers with differing yeasts mashing in a range from 20 to 35 minutes, and in half (3) of those cases, the attenuation suffered significantly. Surprisingly, the attenuation also did not seem to suffer in the other half (the other 3) of cases. I chalk this up to variability in runoff and sparge times, not to mention ingredients, which for me were not constants. But once I started mashing for 40 minutes, all attenuability problems magically disappeared. I have not had one single problem since. I imagine this minimum mash time number is different for different systems, so brewers beware – your mileage may (and probably will) vary at least a little bit.
Here’s something else to think about:
When pro brewers say “we only mash for 15 minutes,” they are probably not figuring in their recirculation time, which can be significant (30-40 minutes). During recirculation, the wort is still at mash temp and conversion is still taking place. After all is said and done, their conversion rest isn’t much less, if at all, than 60 minutes. This is something I discovered while touring Surly Brewery in Minnesota.
I think I heard that somewhere. 
If you have a “hot” base malt with a high degree Lintner rating, you could probably do a quick mash and by the time you got it to mash out temps it would be done. Don’t try this with a base malt with a really low diastatic power (degree Lintner) like Munich, as I had one that was all Munich that was more like 2 hours to fully convert last year. YMMV.
This is an interesting topic, but I find plenty of productive things to do during the mash time.
Is this total mash time? I never look at the total mash time but pay more attention to the temperature ranges I’m at for the styles and final body of the beer I’m brewing. We don’t use a “one size fits all” mash.
We’ll spend 10 minutes at 121°F for a decent dough-in with a very low water/grain ratio. Just enough to get everything wet. Then we’ll infuse up to 131°F for under-modified malts/wheat/rye for 20 minutes. From there we decoct up to 154°F for 45 minutes. However, we might change the mash and cut out the 131°F protein rest and go to 148°F and hold there for 30 minutes and raise up to 158°F for 15 minutes. Again, depends on the style and how we want the body to come out. Then we decoct to 165°F and mash out with 175°F water very slowly. Sparging takes at least 30 minutes.
So our total mash time is over an hour, but rather than focus on that we’re more focused on the temperature ranges, the amount of time we spend there, and what goodies we’re extracting from the grains.
This topic somes down to “What malt are you mashing”. I have a copy of the 1999 special edition on Lagers (doing some reseach on lagers), and Greg Noonan had an article titled “Secrets of the Double Decoction”. In that he states that some malts like dark munich should have a decoction, and fully modified British malts should not be subjected to lower rest temperatures, just a single infusion.
One can select the mash profile for the malt. Use the right tool (profile) for the job.
The duration of mashing is highly dependent on the mashing temperature as pointed out previously in this thread. Figure 9.7 in Malting and Brewing Science shows that at a 150F mash temperature, the degree of extract peaks out at about 60 minutes of mashing time and flat-lines from that time onward. That figure also shows that a 15 minute mash at 150F provides 90 percent of the extract potential.
The figure also compares the extract performance at 120F. Since this is below the starch gelatinization temperature, its not surprising that the extract potential is lower than for the 150F mash and the time to get that extract is also much longer.
Figures 9.9 and 9.10 from that text also illustrate the time and temperature dependence of mashing. My interpretation of the figures does support the contention that at typical 150F + mashing temperatures used with modern well-modified malts, a short mash is OK and extending the mash time doesn’t really get us anything more. I was there for the 2 Brothers mashing presentation at the NHC and heard that contention that short mashs were OK. I did not know that it was the subject of a PhD dissertation, but I am even more inclined to believe that short mashes can work now.
Another extrapolation of this time/temperature trend suggests that those of us performing a 160F + mash out are even less likely to need long mash time. I’m thinking that my future mashes will be much shorter.
Thank you for not typing “YMMV”, man that annoys me 
I usually mash for 60 minutes and I always seem to get crazy attenuation no matter what my mash temp was. I’ve had 80+% attenuation with a mash temp of 156F, of course that could be because I used US-05, but still. Maybe I’ll try mashing for 45 minutes on my next batch for kicks.
There’s some great information in this thread.
I haven’t seen it mentioned, but the crush plays a role in mash times. Coarser needs longer and vice versa. The malt also needs to be properly hydrated (dry malt won’t convert). The diastatic power of the grain is a factor; if you are doing a Munich-heavy batch, or using a lot of adjuncts so that the effective DP of the mash is low, then it will take longer. Different enzymes work at different rates, so the mash temperatures used to achieve the target wort composition will also play a role.
If you grind your grain to a flour and use high DP malt, then it will likely convert in a handful of minutes.
Personally, I don’t find conversion speed the variable upon which to optimize my brewing process.
If you think you’re mashing too long, do a starch test and find out.
On my 4th all-grain batch, I had to mash for 70 minutes, until an iodine test showed full conversion.
…Not sure what happened, but that oatmeal stout turned out awesome!
I think in the end, what enzymes do what work and how well they access the starches and proteins in the grain is what will determine fermentability. I used to think that it could happen REALLY fast, but I’ve since been convinced that resting a little longer to make sure all the starches have been liquified and converted makes some sense. Plus, I am not really worried about the extra 20 minutes or so I sit and wait for my mash to finish.
I do know that thicker mashes will tend to provide the enzymes some thermal insulation and allow the lower-temp enzymes (protease for example) to work a bit longer and thinner mashes will then not favor those lower-temp tolerant enzymes but be more partial to the enzymes that have greater heat tolerance.
I want to read Kai’s info again…didn’t he discuss this at NHC too?