ZydoSEO - 3:28 am on Jul 30, 2013 (gmt 0)
Pretty much the only time Google will show your meta description is if all of the keywords in the user's search phrase are found in the meta description. It's why I optimize my title element first (should contain the 1-3 keyword phrases your URL is targeting and most likely to rank for if you're doing your job well).
Once I know what my title element looks like I then optimize my meta description so that it uses hopefully most if not all of the individual keywords from the title. This should maximize the percentage of time that Google actually displays your meta description as the snippet.
The only time Google seems to show more than 150 or so characters from your meta description is if the search phrase it ranks for is a very long tail keyword phrase (typically 5 or more keywords in the phrase, maybe 4+ in some instances). Then and only then might they show 200, 300, even 400 characters from the meta description.
So if you're targeting primarily 1-3 word phrases, then I would stick to 145-150 as the max length to avoid the unattractive elipses [...] at the end where Google truncates it. If your going after very long search phrases then you might consider something longer, though 150 characters is almost always enough to have 1-2 sentences, each starting with a call to action and using most if not all keyword phrases from the title element. So I honestly have never needed more than 150.