This is what Matt Cutts told Stephan Spencer when asked about the optimal length of a keyword-rich URL (web address). Excerpts:

Matt Cutts: If you can make your title four- or five-words long - and it is pretty natural. If you have got a three, four or five words in your URL, that can be perfectly normal. As it gets a little longer, then it starts to look a little worse. Now, our algorithms typically will just weight those words less and just not give you as much credit.

Stephan Spencer: So, would something like 10 words be a bit much then?

Matt Cutts: It is a little abnormal. I know that when I hit something like that - even a blog post - with 10 words, I raise my eyebrows a little bit and, maybe, read with a little more skepticism.  Link.