It depends on your sites importance to Google. Some sites will be crawled more frequently than others (sites like BBC and Youtube are crawled daily), but most can be between 4 and 6 weeks
However, when Google re-crawls your site it will still not necessarily display your meta description in the results. Whether your description is displayed or not, like dangergeek says, will depend on it's relevance to the search.
If Google can find a snippet more
relevant to the search within your page content, then it may well end up displaying that instead. A short read on
search result snippets explains a few factors.
Also, make sure your meta descriptions reflect the page content, with a
fluid description. You've got about
160 characters to work with, so make the most of it!
What about something like this?
Also make sure each post has it's own description (All In One
SEO plugin for Wordpress can do just that)