Hopefully, I asked that correctly, once the yeast is pitched within a number of hours it begins to consume the fermentable sugars in the wort, create CO2, etc. once fermentation is completed, if I'm correct, it begins to "clean up" undesirable chemical components, I understand some yeast do this quickly, some not, what would be an approximate period of time for the clean up to be completed? Following that process does the yeast just go dormant and settle to the bottom of the fermenter. Please feel free to correct my understanding as need be, thanks.
Yeast goes through an aerobic (oxygen consuming) and anaerobic (non-oxygen consuming) life cycle in your fermenter. The first phase is the one in which the yeast multiplies and becomes fruitful. During this time, very little CO2 and alcohol are produced, but sugars and proteins are getting consumed. The cells are spending all their time making more cells.
Once all the oxygen is gone, the yeast switch to anaerobic metabolism. This is the phase where the majority of sugars are consumed and basically all the alcohol and CO2 are produced.
As the yeast begin to run out of fermentable sugars (e.g. small sugars) and fermentation comes to an end (a relative term here), they become more and more desperate to find food. This is when more complex metabolic pathways are activated and some "scrubbing" can take place. This is also what you would typically refer to as the secondary and conditioning. Basically, the yeast are beginning to starve and are trying to eat less and less palatable sources of energy including some of their own wastes (e.g. diacetyls).
I'm not aware of any really firm data on what "enough" time is in this last phase. Basically, it's part of the art of brewing. You want to have enough time in the secondary to let your flavors mature and part of that is the yeast searching for food. Part of it is other obscenely complicated organic chemistry (read: black magic). For many beers, this is a week or two. Typically, the higher the OG, the longer you will want to condition.
Yes the yeast does go dormant and starts to settle. If you add more sugar it is possible for the yeast to reactive and start fermenting again.
This is what happens during secondary fermentation.
Hope this helps