Maria: There is no direct correlation between conching time and quality across machine types. Modern conches are built in a way to produces very high quality results in surprisingly short periods of time.
How long you conche depends on a lot of factors, based on the flavor profile you are looking to achieve. The beans you use provide the starting point, and proper roasting is the first phase. Different roasts will bring out different flavors which will be affected differently by the conching. When starting out, check every hour for the first 4-8 hours and you will be astonished at the change. From there, every couple of hours for the next 24-36 hours (if you are using one of the CocoaTown machines or Santha, or longitudinal conche).
The challenge is to get the flavor you want in the same time frame as getting the texture right. You can overdo either quite easily.
The key point is that 72 hours (or 96 hours) is not better because it's longer. It's very possible that at 72 hours all of the "life" in the chocolate will be evaporated out. As a chocolate maker, it's up to you to determine what results you're looking to achieve and to experiment until you hit upon the combination of times and temperatures that deliver the results you want. You want to conche for exactly the right amount of time that is correct for the chocolate you are making. It may be different for different chocolates.
One thing you can do to affect conche times is to blow (warm) air into the conche bowl. This can accelerate the evaporation of undesirable aromas (such as acetic acid). This can shorten conche times by getting flavor development ahead of texture development.
clay - http://www.thechocolatelife.com/clay/