Hi,
I think this is a good question, and one of those where you should think on it from a user perspective.
First of all, you want to have a URL that is:
A) Easy to remember for the user
B) Does the best possible effort of describing the content on the page, so that it's obvious to the user what the page is about, even before clicking the URL.
In this post, Rand says that the URL should be < 90 characters:
http://moz.com/blog/visual-guide-to-keyword-targeting-onpage-optimization
Moreover, on this page you can see that there's a limit on URL characters in Internet Explorer(4-8, but there's also some limits in newer versions) which is 2083 characters:
http://moz.com/learn/seo/url
There's also some additional good information on that page, for your reading. For more information on the length from a technical standpoint, I'd recommend the top answer in this thread:
http://stackoverflow.com/questions/417142/what-is-the-maximum-length-of-a-url-in-different-browsers
This blog post indicates that most of Google's urls are around 70-90 characters, which may be a good indication:
http://www.johnfdoherty.com/lessons-from-google-about-url-lengths/
As a final note, SERPs seems to only show around 50 characters in the URL fields, so this may also be something to keep in mind, however, they do tend to highlight the keywords and shorten the rest of the URL (for example www.google.com/.../insidesearch/howsearchworks/...).
To summarize, I do not think there's a "definite best practice" defined in terms of URL length, as a general rule of thumb in these scenarios; as long as it makes sense to the user, it should make sense to the search engines too. I hope I was able to help somewhat.