While Google has recently been showing search result snippets featuring long meta descriptions (around 320 characters) over the past few months, Google have once again changed the rules and snippets are now back to their old length (between 150 and 170 characters).
Google’s Danny Sullivan confirmed that Google, in fact, has changed the meta descriptions. However, he doesn’t say how long the new meta descriptions will be. He says that the length will be shorter, but variable. Research of Moz shows that most meta descriptions are roughly the same length as they were before Google decided to increase the number of available characters.
Don’t panic though. If you have meta descriptions longer than 155 characters, we’d advise you to make them shorter, at least for your most important pages and posts. In most cases, Google will not show the long descriptions in search results anymore and you don’t want Google to cut off your page meta descriptions in the middle of a sentence, or worse, before you get to the essentials of what your page is about. So you’ll probably want to rewrite those long meta descriptions, making them fit Google’s new rules.