How long does it take for a beer to go flat?

There are many variables that contribute to a beer going flat (and by flat, we mean lose carbonation). If you pour it into a mixing bowl, it will go flat faster than if you leave it in the can or bottle because more of it is exposed to air. The type of beer is also a factor, as some contain more carbonation or are denser than others. According to opinions on the internet, the time period ranges from 30 minutes to 48 hours for a beer to go flat.