I see this question alot in the homebrew forums on-line: “How long should I leave my beer in secondary?”
This depends on a lot of things. The reason most people do a secondary fermentation step is to get more stuff to settle out of the beer. If you use a glass carboy for this (preferred) then you can visually check the beer on a week to week basis. Once the level of junk at the bottom doesn’t change much, you are good to go. I think in most applications 2-weeks should be plenty to help clarify a beer.
The second reason is to “bulk” age or maturate a beer. Again this is very recipe dependent. If the beer is not a high alcohol beer then it will mature faster than a big monster beer.
Lastly, you really will just need to experiment with maturation times. You get a sense from the bottled product too if longer maturation is needed. Then next time you brew maybe you can add some of that time to the secondary instead of waiting it out in the bottle.
The last thing to concern yourself with extended secondaries is yeast for bottling. If you plan to bottle condition (prime with sugar before bottling for natural carbonation) generally there will always be enough yeast to carbonate. The longer the secondary, the longer it may take to carb. With that said though, if I was to leave something in secondary for 6 months or more… that yeast may be worn out and too weak to really carbonate well, and adding an eighth to a quarter packet of rehydrated dry yeast to the bottling bucket may be necessary.
I hope that helps clear up some of that issue.
Check out these other threads of ours regarding secondary (and primary) fermentation: