We need to clarify something for you...Your beer doesn't ferment for x number of weeks. Beer only ferments for as long as the yeast takes, which is usually about a week. And you determine that with 2 consecutive hydro readings.
The rest of the time a beer is left alone is for conditioning
whether in extended primary OR a secondary.
You don't CHOOSE how long a beer ferments for.....you let the yeast do it's job...you want a beer to finish fermenting.
Now as to how long after fermentation is complete you want to leave your beer for, This is the most discussed topic on here, it has been readily covered. I suggest you read THIS thread, it's become the "uber discussion" on this topic thread.
To Secondary or Not? John Palmer and Jamil Zainasheff Weigh In .
Many of us leave our beer in primary for a month minimum then bottle.....We find out beers to be clearer and better tasting.
I suggest you read that thread, and experiment for yourself, and make up your own mind.
There's thousands of threads where folks have ventured their opinions, and argued incessantly, but it ultimately comes down to what works for you......me, it's a minimum one month before I bottle.