Your question can be answered in a few ways..... technically a beer can be fully fermented in a few days but by no means is it "ready". what I mean by this is that by leaving the beer on the yeast cake for a few WEEKS it will clean up after itself reabsorbing any off flavors caused by fermentation it also will flocculate further resulting in a clearer finished beer also, the flavor will be ultimately far better. So technically yes your beer can be done in a week or less but the finished product will not be as palatable. Patience is key when it comes to brewing.
Funky Onion Brewing est.2010
Primary-Turbid mashed Lambic
Secondary-Burley whiner American barleywine
Primary-A dark German lager or a Hoppy Munich Helles