How do you know it's stopped fermenting? Have yo taken a gravity reading?
If you're talking about your airlock stopping bubbling, then all you know is that your airlock has stopped bubbling. It could just as easily be bubbling or stop bubbling for that matter, due to changes in barometric pressure, temperature, or whether or not the cat or vacuum cleaner bumped into it, as it could be to because it's still fermenting.
Bubbling in primary SHOULD slow or stop eventually...Bubbling stopping or slowing down doesn't mean fermentation has stopped it ONLY means bubbling has. An airlock is a VENT, a VALVE for EXCESS CO2. It's not a magic fermentation gauge. When the majority of sugars are eaten in the initial burst of fermentation, lots of co2 is released. As it slows down, bubbling ceases or stops altogether because there's not as much EXCESS Co2 being released.
But that doesn't mean fermentation is over, just that it's slowed down.
Fermentation is not always dynamic...just because you don't SEE anything happening doesn't mean that the yeast aren't happily chewing away at whatever fermentables are in there....the only way to know comes from gravity readings, and nothing else.
Activity, action, bubbles, even krausen can be affected by the envoironment just as much as it being caused by the yeast...so going by that is NOT reliable.
If you want to know what's going on with your beer, then take a gravity reading. The only way to truly know what is going on in your fermenter is with your hydrometer.
Like I said here in my blog, which I encourage you to read, Think evaluation before action
you sure as HELL wouldn't want a doctor to start cutting on you unless he used the proper diagnostic instuments like x-rays first, right? You wouldn't want him to just take a look in your eyes briefly and say "I'm cutting into your chest first thing in the morning." You would want them to use the right diagnostic tools
before the slice and dice, right? You'd cry malpractice, I would hope, if they didn't say they were sending you for an MRI and other things before going in....
Counting bubbles does not equate to anything usable in fermentation. It's not like "x bubbles/minute= y gravity points." It just means that co2 is being released....but it could also NOT be bubbling, and still fermenting away.
Nowadays many of us opt for a month long primary rather than racking to a secondary, though when I do secondary, or recommend it to folks I tend to do a 2 on 2, two weeks in primary to let the yeast clean up after themselves, then a minimum two weeks in secondary.. But I don't recomened anyone rack to a secondary indiscriminately based purely on a calendar.
It should be done based on 2 gravity readings over three consequetive days to insure the beer is done, if one was considering racking on day 14, then I would take a reading on day 12, and then 2 days later- if the numbers match, the fermentation SHOULD be complete, and racking can commence.
But like I mentioned in most situations we skip secondary and leave it in primary for a month or more instead. The reasons and benefits for polonged yeast contact have been well documented hundreds of times on this forum....
This is the best dicussion on the topic to be found on here. You can read all the thoughts on it there.
To Secondary or Not? John Palmer and Jamil Zainasheff Weigh In .