If you conducted a proper fermentation, there's no reason to leave it longer. Here's a quote from John Palmer that will appear in an upcoming BYO article I wrote...
"Yeast have 3 phases in their life cycle: Adaptation, High Growth, and Stationary. (See Yeast by CW and Jamil) They do not have a maturation phase where they clean up byproducts. Adaptation phase is where they take in oxygen and build sterols and other lipids, assess the sugar composition and build enzymes, etc. Once those activities are done, they start the High Growth Phase, eating and reproducing. The number of cell divisions is limited by their lipid reserves they made during Adaptation. These reserves are shared with each daughter cell. When those lipid reserves are exhausted, the cell stops reproducing. In addition, when those reserves are exhausted, the cell is old and cannot eat or excrete waste efficiently across it’s cell membrane. A yeast cell typically can reproduce about 4 times during a typical fermentation, after that it is old and tired and tends to enter Stationary phase where it shuts down most of its metabolism and flocculates, waiting for the next batch of aerated wort. Stationary phase is essentially an inactivity phase, resting on the bottom.
Like I said, no conditioning phase as far as the yeast are concerned. Byproducts can be consumed at any point during the high growth phase, but they are a lower energy source than sugar, so guess what? Byproducts are not a biological priority. The brewer therefore needs to plan his pitching rate and fermentation conditions such that the yeast run out of fermentable wort sugar before their lipid reserves are exhausted and they go into stationary phase. Now you have a majority of vigorous yeast that have only undergone 2 reproductions (for example), the sugar is gone, and they are still hungry, so they turn to acetaldehyde and diacetyl as alternate energy sources and maturate the beer. You can help this by doing a diacetyl rest by raising the temperature a few degrees after the first half of fermentation, to keep the yeast active and eating. Where in the fermentation? after the first half, 2/3 to 3/4, when most of the attenuation has occured and raising the temperature is not going to cause rampant growth and the off-flavors associated with it. "
