Fermentation Times

How does one determine the length of fermenting?  What determines the length of primary fermenting?  I understand if the beer is in the secondary a gravity reading will determine when its finished.
An example of a wide range of fermentation times is Northern Brewer general rule is 2 weeks in the primary and weeks or months in the secondary.  I have seen BrewSmith state 4 days in the primary and 10 days in the secondary.

Any thoughts would be appreciated.

Thanks!!

I once asked one of my chefs in cooking school how long to cook some particular item, and his reply was "Until it's done."

I know that sounds like a smart-ass answer, but it's the truth. How long do you cook a steak on the grill? It depends, right? How thick is the steak? Is it cold or at room temperature? How hot is the grill? Will the grill be open or closed? What cut is it? It's not a question that can be given a definitive answer.

How long do you leave it in the primary? Until it's ready to go into the secondary.

I wait until the krausen has settled and the beer starts to clear. How long does that take? An ale that takes off right away with a quick and vigorous fermentation could be ready in less than an week. A high gravity ale could take ten days or more.  A lager might take three times that.

The best answer I can give you is the same answer my chef gave me: until it's done.

+1.  And these days, it is not just "finished fermenting" that many brewers wait for.  Many brewers now extend primary a week or two for clarity and go straight to packaging.

Generally, I wait 3-4 weeks until it's acceptably clear, and then I keg.  At 45F in the fridge, the rest will drop and the first pint will usually purge the goop. 

I agree with all of the above.  My process for managing fermentation is to keep track of what the yeast are doing. 

Primary:

I watch what's going on, both visually (churning, krausen formation (krausen rise, maximum krausen and krausen fall) and via gravity readings. 

Knowing what my attenuation should be gives me an idea of how quickly the fermentation is occurring.  Depending on the yeast strain and the flavor profile, I might start ramping the temperature up towards the end of the primary phase (for example, when I've reached 80% of how far it will ultimately attenuate.  This ramp up in temperature, will cause the yeast to become more active (due to the warmer temperatures) and clean up after themselves.  This gets rid of some unwanted estery flavors.  The main flavor profile was already established in the first 80% of the fermentation. 

Once it's been ramped up and finished it's attenuation, then I'll drop the temperature back down and let the yeast slowly precipitate out and settle.  When it's nice and clear, I then keg or bottle.

As for secondary fermentation, I only do this when I'm adding something, or I'm going to be aging it a long time.  Examples of why I would secondary are:

• Adding Fruit or other stuff after primary.
• Making a sour or a bret beer that needs long conditioning times.  I don't want to leave these on a large yeast cake for extended periods of time.
• Lagers that will be lagered for extended time.  I don't want these on a large yeast cake for extended periods of time either.

My best advice is to be patient. Yeah, gravity might be where you want it after 3,4,5 days...but sometimes it takes additional time for the yeast to "clean-up" and for the flavors, smells, etc to "meld".

Mark