Update: Google has since reverted the length of the meta descriptions back to the original 120 - 170 characters.
For years, SEO specialists have been telling that the perfect length for a meta description is around 150 characters. That is based on the space Google allows in the SERP: using a maximum of 170 makes your meta description fit perfectly in the little text snippet.
Recently Google has given the snippet a bit more space. As a result, you can now fit a longer meta description in the SERP. In their own words:
“We recently made a change to provide more descriptive and useful snippets, to help people better understand how pages are relevant to their searches. This resulted in snippets becoming slightly longer, on average.”
Based on research from Search Engine Journal, the maximum length seems to be above 350, although it varies on the search query. We've updated the meta description checks to allow meta descriptions up to 400 characters, instead of the previous 170 upper limit.
Although meta descriptions are not used for ranking your page, it is the ideal place to entice searchers to click to your website. You could expand the meta descriptions of your most important pages, but Google does not recommend updating all your meta descriptions to use the extra space.