Update (May 25th, 2018):
As of May 2018, Google has reverted back to shorter display snippets. Data collected from Moz suggests these changes are widespread and that most meta descriptions are now being shortened to the previous limit of about 155–160 characters. Given this new development, consider this post a record of Google's experimentation with snippet length, which, as of right now, does not seem to apply.
Jump to the bottom of this post for updated recommendations on meta description length.
In early December 2017, Google officially confirmed it has lengthened the snippets of text that appear beneath links in search results.
These description snippets have remained around 160 characters for many years, but a recent report from RankRanger has measured average growth to be around 230 characters, with roughly half of users seeing these changes as of early December.