Primary. How long is too long?

I ferment pretty cool, sometimes dipping to 60-61°F where I keep it, so 4 weeks is common for me for most of my beers. I have only gone longer deliberately with bigger (1.080+ OG) beers.

Temperature and whether you are in plastic or glass could have an effect on whether your beer is getting oxidized or stale, but you shouldn't have any souring unless you have a sanitation issue, even if you let it sit for 2-3 months.

 

There doesn't necessarily have to be any great hurry in many cases, but the amount of trub that may be in the fermentor can be a deciding issue so that you don't start picking up vegetative flavors. If it is a hoppy beer, then you're missing out on enjoying a fresh, hoppy experience the longer that you wait.

 

as long as your temp is actually steady at 68F then your risk of leaving the beer on the yeast for many weeks is minimal if you used good healthy yeast and pitched the proper amount. That being said if it gets warmer, or you under pitched, or you used unhealthy yeast your risk increases the possibility of unwanted flavors due to higher amounts of esters, phenols, autolysis. with healthy and the proper amount of yeast undesired flavors are usually cleaned up by the yeast if kept in a good, stable environment and the beer can condition just fine without transferring to secondary for a good long while. just have to keep the controllable factors in check and the beer will be fine.

 

10 weeks is the real limit in my experience. Before that your beer will be fine.

 

As others have said I think it depends on the beer style, I wouldn´t let an Ale beer primary go further than 45 days regardless what temperature are you fermenting at. I had a bad experience with a delicious coriandered belgian beer,I took a sample on 3rd week fermentation and it tasted wondeful , a week later it has lost almost all that precious flavor turning into a lower quality beer, taking into a count I was(as always) very careful not to introduce oxygen nor contamination.

 

A few weeks ago I would have posted that waiting 4+ weeks is OK but a recent post by @honkey has given me pause: “I can just about guarantee you will be happier with your beer if you pitch healthy yeast and get the beer off the yeast cake soon after letting the yeast clean up off flavors.”

And: “I have witnessed pH rise as much as .1 in a day from autolysis. Early on, you may not taste autolysis in its typical flavor descriptors (meaty, rubber, savory, tar, etc.) but what will happen is that pH will rise to be closer to the pH of your saliva. When that happens, flavors become less discernible and they can become more muddled.”

http://www.beeradvocate.com/community/threads/took-beer-off-yeast-accidentally.256372/#post-3275586

FWIW, it is my practice to give the yeast a few days to perform its ‘clean up’ activity and then promptly bottle.

Cheers!