I have been getting this call about ten or twelve times each week recently, so that usually means it's time for an article.
How long should it take for a beer to ferment? (When I say "ferment" I mean the time it should be in the primary fermentation vessel.) Unfortunately, there is no set answer to this question, but there are some solid guidelines. We'll start with the most basic.
The dried yeasts we sell at Main Street are extremely fresh and vigorous. Therefore it is not unusual to see a beer ferment completely - from start to finish - within one to three days. In fact, it is very unusual to see a beer fermented with our dried yeast go longer than four days. So listen, me droogs: most beers fermented with dried yeasts will be finished in three days. In the summertime, when temperatures are warmer, we have seen beers completely fermented out in only eighteen hours. Done. Finished. Ready to rack. Eighteen hours.
If you have used a dried (dehydrated) yeast, and you have seen crazy activity for a few days and then the activity has stopped or slowed down, you have a normal fermentation. Rack off into secondary when the activity drops to one bubble every ninety seconds.
Liquid yeasts are different. Beers fermented with liquid yeasts will take longer to start (usually you will see no activty for twelve to eighteen hours), and - once activity begins - they will usually ferment for eight to fourteen days, slowly and steadily. Some beers, especially when they are fermented during the hot summer months, might finish with five days. Higher alcohol beers, or those fermented in cooler (60 degree) environments, might ferment for 21 days. We sell a liquid Belgian witbeer yeast that almost always takes 21 days to finish, no matter what conditions it is in.
Lagers, again, are different. Lagers are fermented in the cold (45 to 55 degrees) and that will slow down the activity rate and prolong the fermentation time. Lagers can often stay in the primary fermenter - slowly bubbling - for three or four weeks. This is good. Do not rack lagers early or you run a very high risk of stalling them out and leaving them overly sweet. Leaving lagers in primary for long periods of time is a good thing - do not panic. At the end of the lager fermentation - usually always after two or more weeks - you can warm the beer up to 60 degrees in order to finish the activity. And then rack off to secondary. See our lager page for more info on cool-temperature fermentations.
If you are in doubt about whether your beer is stalled out prematurely or not, you can always do a hydrometer reading. A finished beer will usually read 1.010 to 1.020 on the specific gravity scale. Stalled out fermentations are extremely rare. I only hear of them two or three times a year among all my brewers, so usually when a person thinks their beer is stalled, it has just fermented very quickly. Which is pretty normal. If you need to call for advice about this, I almost always will be able to help you more if I have a current hydrometer reading.