Fermentation is the only part of a brew day you cannot rush and the only part you can measure while it is happening. The airlock tells you almost nothing (it stops bubbling long before the yeast stops working and it bubbles enthusiastically when a lid seal leaks) but the gravity tells you everything, because every degree the beer loses is sugar that has become roughly equal parts alcohol and carbon dioxide. Learning to read a ferment means learning to ignore the airlock and take a reading.
What the curve normally looks like
An ordinary ale pitched at the right rate does most of its work in three days and then slows sharply. A beer that went in at 48 degrees will typically be in the low twenties after 48 hours, in the mid teens after four days, and then move a degree or two over the following week as the yeast finishes the last, slowest sugars. That long tail is where beers get finished properly, and it is why a beer bottled on day four and a beer bottled on day ten are different beers even when the airlock stopped on the same afternoon.
Finished, or stalled?
There is exactly one test: two readings two or three days apart that agree. A gravity sitting at 20 degrees when the recipe said 11 has either finished early or stalled, and the two look identical in the jar. Warmth and a gentle rouse will usually restart a stall; nothing will restart a beer that has genuinely run out of fermentable sugar because it was mashed too hot. Either way, packaging a stalled beer is how bottles come apart, because the yeast will finish the job eventually and it will do it inside sealed glass.
What pressure fermentation changes
Fermenting under a few tenths of a bar of back pressure suppresses some of the esters and higher alcohols yeast makes when it is warm, which is why it is used to ferment lager-like beers at ale temperatures. It also changes something arithmetic: the beer finishes in equilibrium with a much higher partial pressure of carbon dioxide, so it is already carrying considerably more gas than an airlocked beer at the same temperature. Prime it as though it were flat and the bottles will be over-carbonated. The priming calculator here has a box for that pressure for exactly this reason.
Lagering, which is not fermentation at all
Lagering is the cold conditioning that comes after the ferment has finished: weeks near zero, during which yeast and haze drop out and the flavour settles. The gravity does not move, so there is nothing to measure and nothing to wait for except time. Two things follow. Lagering is not a fix for a beer that stalled (cold makes a stall worse, not better) and the residual carbon dioxide the beer carries into packaging is set by the warmest temperature it saw after fermentation finished, not by the cold store it spent a month in afterwards.
Questions people ask about beer fermentation
How long should fermentation take?
An ale is usually done in seven to ten days at 18 to 20°C, a lager takes two to three weeks colder. Both numbers are averages and neither is the answer: two matching readings are.
My airlock never bubbled. Is the beer ruined?
Almost certainly not. A lid that does not seal lets the gas out around the rim, and the beer ferments perfectly well. Take a gravity reading: if it has dropped, it is fermenting.
Should I move the beer off the yeast partway through?
Not while it is still working. The yeast still in suspension is what finishes the last sugars and cleans up the compounds made during the fast phase. Racking early is a common cause of a beer that stops a few degrees high.
Does a warmer ferment make more alcohol?
Not meaningfully. It ferments faster and often a degree or two further, but the strength is set by how much fermentable sugar was in the wort. What warmth reliably changes is flavour, and mostly not for the better in a beer meant to be clean.