How long should Meta descriptions be?

More than 120 words and less than 150 words is best for seo.

Metas now a days are not worth to worry about as search engines do pick text from content randomly. Also for content length, sometimes google show less and sometimes more (around 250-300) so any text length between 100- 160 is good. But don't waste much time on writing about meta, just add casually and related with/wo keywords like if you write about Wordpress plugin do add meta like check top Wordpress plugins to charge your site. Dont try to manually make it long, just keep information much required.

In this, you should tell about the blog, on which topic you created your blog and make a short description of what information you are going to give to your readers through the blog, which should be in 150 letters, there will be no use in writing more words because search engine does not show more words than this on search pages.