Primary fermentation includes a period of active fermentation that lasts about 48-72 hours and is considered done when hydrometer readings are stable.
Typically, the active fermentation stage of primary fermentation will last between 2-3 days and the beer will stay in the primary fermenter for 1-4 weeks.
After primary fermentation, the beer will either be moved to a secondary fermenter, bottled, or kegged. Simple ales normally do not require secondary fermentation while larger, more complex beers may need a secondary fermenter to help with conditioning, dry hopping, flavor additions, or clarity.
Active fermentation normally starts within about 12 hours of pitching the yeast and it will last about 48-72 hours from that point.
Variables such as beer recipe, yeast strain, and fermentation temperature will all impact the length of active fermentation.
When all signs of active fermentation have ceased, yeast has fallen out of the beer, and you have stable hydrometer readings for several days in a row you can be sure that your primary fermentation is complete.
Here are visual signs that fermentation is complete: – There will be little or no bubbling from the airlock. Some bubbling is normal, even after fermentation is complete, as carbon dioxide periodically escapes from the beer liquid. – All or most of the foam will dissipate from the headspace of the fermenter.
You will know that your fermentation is done with a hydrometer when the readings are the same for multiple days in a row and they are in line with your recipe’s estimated final gravity reading.
Technically speaking, you can’t ferment a beer for too long because once the sugars have been eaten up active fermentation will end and there will be no danger of the process continuing past that point.