I know this might sound flippant, but it's not meant to be. Try putting some searches into Google to see how many characters get returned. Or if you get more traffic from Bing vs Google, do the same exercise on Bing. I don't think there will be a lot of difference between them.
I count the characters with my Word Processor (it's just a copy and paste, and then a hot key) and Google returned anywhere between 160 to 260 characters.
The keyword I entered was "dry rub barbecue cooking"
This is the number 4 result on page 1.http://www.epicurious.com/recipes/food/ ... rub-242244
That's 265 characters, but it's an exception. Most are around 160, maybe up to 200 characters in length.
If you look at the View Source, Google has made it's own description up for that entry because the one in the code is
"OK, this is ground zero—the ur American basic barbecue rub. Use it on ribs, pork shoulders, chickens—anything you want to taste like American barbecue."
Google didn't like that, and supplied its own description instead.
So, it's not set in stone. The basic idea is to write a Description that will entice the visitor to click through. That's it, the number 1 reason for a description.
Do some checking in the Google SERPS to see what you find.