When we are developing an application in a team environment, it is important to have a well-defined coding style that everyone on the team adheres to. This allows us to have a code base that is easy to read and maintain.
If a style guide remains static for too long, it means that it is probably not keeping up with the latest changes within the language. What is too long is different for each language. For example, with the C language, too long will be defined in years, since the language is very stable; however, with Swift, the language is so new and changes are coming pretty often, thus too long can probably be defined as a couple of months.
It is recommended that we keep our style guides in a version control system so that we can refer to the older versions if need be. This allows us to pull out older versions of the style guide and refer back to them when we are looking at older code.