Fermentation Time Line
Yeast goes through a few different phases after it is pitched into a fresh batch of wort. It is important to understand what yeast is doing in these phases so as to be able to control the fermentation and fine tune a batch of beer.
1) Lag Phase. This is the phase where there is no apparent activity, while the yeast is acclimating to its new environment. It is important to stress that this phase is important for creating healthy yeast. Yeasties are uptaking minerals and amino acids from the wort, as well as vitimins and minerals. This phase should last from 3-15 hours, which I call the target window. If fermentation begins too quickly the yeast will not be as healthy at the end of the fermentation. During this phase, there is little in the way of flavor or aroma compounds produced so it can be started warmer than the fermentation temperature (allowing for a quicker start). Of course the temperature will need to be reduced to the proper fermentation temperature thereafter.
2) Exponential Growth Phase. This phase occurs 1-4 days after the lag phase. Yeast begins to digest and convert sugars to alcohol, and multiplies in number exponentially. Foam forms on the surface of the wort and an olive like aroma escapes from the airlock. The yeast consumes sugars in a specific order, starting with glucose, followed by fructose then sucrose. Maltose is the largest componet of sugars in wort, and provides the larges flavor component. This sugar is broken down by yeast into glucose components then metabolized. Finally, maltotriose is fermented, but depending on yeast strain, the amount of consumption varies depending on how floculent the yeast strain is, with more floculent strains consuming less. The attenuation rating of yeast is also determined by how much maltotroise it will consume. This phase also has the height of activity phase of the yeast, which is called "high kraeusen", where the foam on top of the fermentation turns yellow to brown, which is primarily precipitated hop and malt components. Brown spots form from oxidized hop resins.
3) Stationary Phase of Yeast Growth. This takes place usually in three to ten days. At this point yeast growth slows down. Most of the flavor and aroma compounds have been produced, but the flavor balance has not yet been reached. This phase is where beer matures. Yeast will re-absorb any diacetyl present. Hydrogen sulphide escapes from the beer and the kraeusen falls, along with the yeast flocculating or settling out.
This whole process is best observed through glass, which is what I always use as fermentation vessels. It is helpful to be able to see the amount of activity and when it appears to have come to an end, transfer to secondary. If fermentation is complete, the yeast will settle out quickly and no further activity will be seen. If there is some fermentation left to be done, there will be yeast swirling and a bit of foam will appear on the surface. Leave the beer until all activity ceases, then package (keg or bottle). Using glass and a flashlight, coupled with eyes on beer, the need for constant testing with a refractometer may be eliminated.