I'm going to brew a batch of beer today. I'm not sure how long I should keep the beer fermented. What is the maximum amount of time, that home brew beer can ferment without it going bad ???
Most beer fermentations are over in about 10 days. if the temperature is a bit cooler then many prefer to leave their brew for 14 days. Some leave it for 3 weeks and some even lager their lagers for 3 months at temperatures as low as 5 degrees. But all that is a rule of thumb. The best way to discover if the brew has stopped fermenting is to use a hydrometer. Take successive readings and if the reading is the same for 3 days in a row then the fermentation has essentially stopped. It is useful to take a first reading just before (or after) pitching - then one can calculate the ABV of the beer.
There's so many variables to give a standard answer, but in short the beer is only going to ferment as long as there are fermentables still in the solution. Depending on the type of beer and gravity as well as temperature throughout, the time is going to vary. Many ales fermented at 18-22C temperature should be done with primary fermentation anywhere between a few days to 10 days or so but there is absolutely no harm in keeping the beer in the primary fermenting bin/bucket for three weeks or more.
Answers can and will be able to get much more complex than this and will take into consideration (among many other things) the oxygen permeability of your fermentation vessel, but provided you're not going to be going overboard in terms of sitting the beer forever, I would think the above should be okay.