The exact algorithms that search engines use are closely-guarded proprietary secrets. However, it is rumored that these guidelines are used: The recommended number of characters for the Title Tag is between 60 and 80 and the Description Tag should be limited  to 150 characters. The maximum number of Keywords we recommend  is 25, with the maximum number of characters 874. The site may be disqualified if keywords are repeated more than 5 times.

However more recently, I've heard talk that Google doesn't even use the "keywords" meta tag in their web search ranking anymore. I did some research and found that verified on a posting dated September 21, 2009 on the Official Google Webmaster Central Blog.