I'm looking at brewing an all-grain barley wine in the near future, and I'm aiming for a high ABV (13%-14% ABV). From what I've heard in discussions elsewhere, by single-infusion mashing at a lower temperature (I'm shooting for 148 for about 90 minutes), I am extracting more fermentable sugars and achieving a dryer taste (both of which I want for this style ale). What are the less than desirable qualities in a barley wine that I will suffer as a consequence for mashing at the lower end of the scale? I'm mashing for 90 minutes instead of the typical 60 to try and guarantee I get the pre-boil gravity I'd like, but is 90 minutes excessive? Is there such a thing as mashing for too long? What are the consequences of mashing for "too long" (however long that may be)?
Yes, mashing for longer than 12 hours may not be good for the beer, particularly if the temperature is allowed to drop during that time. (As anyone who has left a mash for that length of time and taken a sniff will confirm!)
In this case with so much grain you could safely go for a 2 or 3 hour mash, to be sure of complete conversion. since the water to grist ratio is going to be lower (a thicker mash).
With a barley wine isn't the major issue going to be sparging not mashing? Mashing is pretty straight forward, but getting that much sugar off that much grain is usually a pain. When I brewed an all-grain barley wine with a friend the mashing was easy. I think he used 90 minutes for mashing as well. Sparging took forever. With a fly sparge and using a refractometer checking the wort every 20 minutes it took us the better part of 4 hours. It was laborious to constantly be making new sparge water and we ended up with a 15 gallon boil for 5 gallons of beer. I think that sparging critical point for a barley wine.
One issue with sparging is getting too many tanins in the wort which will increase astringency. With a barley wine that is a serious consideration as you are milking all you can get out of the grain. Keeping the pH in a reasonable range (http://braukaiser.com/wiki/index.php?title=How_pH_affects_brewing) and not making the sparge water too hot (http://www.howtobrew.com/section3/chapter17.html).
Sometimes, when I mash too long (more than 90 minutes), I get a stuck-mash. Its as if my grain bed gels-up, like pudding or cold oatmeal. I have found that using rice-hulls or a more coarse grind can help reduce the effect.
If you do an iodine test (with idophor) you can determine if the conversion is done. Once it is done, waiting any longer won't make any difference.
I also have heard of something called "turbo-malt" that is supposed to be heavily modified and converts in under 40 minutes.