how long does canned beer stay good?

Answer 1

If kept constantly cool, 3-5 months.
If the temp varies then 2-3 months.

Here is how to read the top secret codes on American Beer Cans:
The way the code works is this. Letters from A-M represent the month of the year. The next four digits are the day and year the beer was first brewed, and the last two letters are the state code where it was brewed. So you may want to stay away from that fifty cent close-out special at the local liquor store marked M0787DE. Oddly enough, Irish brews have clearly marked expiration dates.

How a beer degrades after it's peak depends on several other variables. Temperature variations over the life span of an unconsumed bottle, how much light and what kind of light the beer is exposed to (both before bottling and afterward), even the color of the glass that the beer is bottled all affect the taste of the beer. Different brands and different types of beer react differently to them, but it's generally considered good practice to prevent massive fluctuations of any of these variables.

Beginnings of freshness dating

The Boston Beer Company, maker of Samuel Adams, was among the first to start adding freshness dates to their product line in 1985. For ten years there was a slow growth in brewers adding freshness dates to their beer. The practice rapidly grew in popularity after the Anheuser-Busch company's heavily marketed "Born-On dates" starting in 1996. Many other brewers have started adding freshness dates to their products, but there is no standard for what the date means. For some companies, the date on the bottle or can will be the date that the beer was bottled; others have the date by which the beer should be consumed.

Answer 2

The rule of thumb for commercially bought beers, whether in a can or bottle, is the fresher the better.
As stated earlier, if kept in cool storage and under low light, it will keep longer. Generally speaking though, canned or bottled beer is not meant to be kept for long periods. If kept too long it may be skunked or taste stale.

There are some exceptions though. There's a lot of Belgian or Belgian style beers that are bottle conditioned. Which means they have added sugar and yeast which cause a secondary fermentation in the bottle. Therefore they are suitable for aging per say. But not too long, maybe a year or two max.
Also beers that are massively hopped, like India Pale Ales, keep longer too. Hops are a natural preservative.
And lastly, beers that have a higher alcohol content keep longer. Alcohol also is a preservative.

Answer 3

no more than 6 months under proper temp.