In this question, Basic Brewing Radio was quoted as saying longer mashes (up to 90 minutes) were "better". It seems as if there are practical limitations with maintaining temperature, as well as just wanting to limit the amount of time spend on the process. But setting aside these kinds of things, what would be the longest you would want to mash? In other words, you have a mash tun that holds temperature perfectly and you have all the time in the world. What "bad" things happen, if any, when you hold your mash at a target temperature for 2 hours, 4 hours, 8 hours?
I know a handful of brewers who have done overnight mashes. So 12-18 hours. I have experimented with this as well. My last two beers were done with 12+ hour mashes. One of these beers was a recipe I have brewed before. The resultant beer seemed nearly identical to my normal 60min mash schedule.
The only issue with prolonged mashes like I describe is that if you go to long you'll start to get some bacterial issues in the mash as it cools back down to that 100-120 range. In my case both mashes at 154 were around 146 in the morning. I mashed in a cooler, covered with an old sleeping bag and several towels.
6 hour mash, IMO, will have little to no effect on the beer. Assuming your mash pH is good to not slowly extract tannins.
In short, your mash and conversion happens pretty quickly. After that its just sitting there becoming ever so slightly more fermentable, but not really. So the prolonged mash is fine if you need to mash in in the morning and finish up at night.
(In case anyone wonders, the over night mash was done as a family man's solution to the long brew day. Mash in after kids go to bed, and you can still get up early enough to finish the session before the kids wake up. That way I am not up all night then exhausted in the morning when the wife needs to get to work and the kids want their pancakes.)