If you keep URLs under 2000 characters, they’ll work in virtually any combination of client and server software.
Extremely long URLs are usually a mistake. URLs over 2,000 characters will not work in the most popular web browser. Don’t use them if you intend your site to work for the majority of Internet users.
IE8’s maximum URL length is 2083 chars, and it seems IE9 and IE10 also has a similar limit. You can click a URL which is longer than this, but the address bar will still only show 2083 characters of link.
There’s also some research from 2010 into the maximum URL length that search engines will crawl and index. They found the limit was 2047 chars, which appears allied to the sitemap protocol spec. However, they also found the Google SERP tool wouldn’t cope with URLs longer than 1855 chars.