there is no standart for that. It's better not to use more than 15 word on keywords. And no more than 150 characters as a description. I always do this way. However one of my site's pages has more than 400 charecters either as a keywords and description, no problem at all, all indexed by google.
Google will chop off the description if it is too long. This usually happens at about 150 characters.
meta-tag; keywords and descriptions should not be too long... short and related description and nicely related and limited number of keywords are always prefered by serach engine crawlers.. you can also analyze your meta tags here: http://www.seocentro.com/tools/search-engines/metatag-analyzer.html ..
Like others have said it varies. I'm pretty sure it is done manually for higher ranking pages. If your description or title drags too long they will chop it short