I brewed my first all grain batch and it is a one gallon batch. The fermentation started pretty fast and it was aggressive for about 20 hours and then went silent and the beer is settling already. Is it possible to ferment that quick? seems too fast to me.
Length of time for the initial active fermentation is dependent on the yeast used and the temperature of the beer in the fermentor. The bubbling through the airlock stage is the active part of the fermentation. After the active fermentation slows the yeast will take more time to finish the fermentation.
Worts that are overly warm will promote more active fermentations. The yeast also produces heat as the fermentation progresses so the beer temperature will always be warmer than the ambient temperature. Overly warm fermentations can produce higher levels of esters which may or may not be desirable for the style being brewed. Fusel alcohols can also be produced if the beer gets too hot.