How long is too long in the primary?

There are too many variables to answer this question definitively. But in general, I wouldn't worry about a few weeks. Once you're into a couple months or more is when I would start to get a little nervous.

 

I hope I never have to find out this answer! I typically leave my big beers on the yeast for four weeks then bottle. The longest I have waited is five with no issues. I have heard stories of longer fermentation's-conditioning but am not inclined to try it. Take care.

 

What VikeMan said. I personally leave all my beers in primary for at least 3 wqeeks, depending on when I get around to checking on them.
For a big beer, and you didn't mention the OG on it, more is usually better, first to give the yeast time to do their thing, then clean up after themselves, but also to let the flavors of the beer meld and get together.
Generally, we homebrewers don't really have to worry about autolysis (yeast dying and giving off those off flavors - and it's not really them dying - but someone else can explain that better than me.)
It's more the big commercial brewers, who have tons (literally) of pressure pushing down on the yeast cake that have to worry about that sort of thing.

 

4-6 weeks is almost certainly not a problem. I would make plans to bottle it before 8 weeks just to be safe.

 

According to @honkey, yeast autolysis in homebrewing happens fairly quickly. Below is a past thread exchange I had with him:

JackHorzempa said: ↑

At the scale/dimensions of homebrewing autolysis takes many months; a week or two of bulk conditioning will result in zero signs of autolysis.

Cheers!

honkey replied:

Actually, that is not entirely true. Yeast will die pretty rapidly following fermentation. A bucket with co2 and ethanol is not a very friendly environment for yeast when there are no more nutrients to consume. I frequently look at my small batch yeast under a microscope at work and I have found that to be the case. The autolysis is not showing up in the taste, but as the yeast dies, the pH of the beer rises. The higher pH will affect bottled beers that you might want to age, leading to faster spoiling. This is not a concern for most brewers, but it is not wise to tell someone to leave beer on a yeast cake for three weeks without describing the potential risks as well. That said, if the beer is being drank fresh, I would just go ahead and cold crash it, save the yeast, and reuse. I also wouldn't reuse yeast that has sat in a fermenter for three weeks at fermentation temps... Even if the viability is still ok (which it probably wouldn't be), the vitality will be low.

 

You'll start seeing low level autolysis after about 3 weeks, IF 1) you started with healthy yeast, and 2) the fermentation temp was kept low (68F or under the whole time). If you start with unhealthy yeast or heat stress it, it'll happen sooner. It takes 6-8 weeks to start seeing wholesale autolysis that gives off the soy sauce flavor if you never transfer off of primary. In a big dark beers (especially stouts), this flavor is pretty effectively masked until it gets really bad, but it shows up like crazy in pale ales.

As they autolyze, the yeast release enzymes, lipids, and metal cations that all have a negative effect on flavor. Best way to forestall this if you know it's gonna be a while until you get to it is to cold crash it. The yeast will not autolyze for a very long time if you cool it down to 34-36 (cool fridge temps). You could have your parents throw the carboy in a fridge if they have room and you'll have some peace of mind.

 

Once again I've learned more in one post from @Peter_Wolfe than I could have scouring out-dated and anecdotal "well this worked for me" threads out there. It's BAs like @Peter_Wolfe @VikeMan @JackHorzempa (and many more, of course) that make this community invaluable. Your time and experience doesn't go unnoticed or unappreciated. Thanks, fellas!

Phil