quick question: how long can beer be left in fermenter?

Context please.

 

I probably wouldn't go beyond a couple of months. Noticeable yeast autolysis is typically a very low risk in a homebrew setting, but that depends on several factors. Personally, I wouldn't tempt fate. Also, plastic is somewhat permeable to O2, so risk of oxidation is another reason not to let it go too long.

 

I ferment exclusively in plastic buckets. I don't go beyond 4-6 weeks. I've never tried longer and never wanted to try longer. I've heard people say you're getting oxidation after a couple of weeks. I've never made a cardboard beer and can attest to a barleywine that I made about 18 months ago. I let it sit in the plastic fermenter for 5-6 weeks before bottling. It's still epically good.

 

I made a beer that was in the plastic bucket over five weeks and it came out fine. Normally tho my fermentation time is 2-3 weeks, then into bottles. Although I do have a glass carboy, I haven't done any extended aging on beers yet.