With all the talk about title tags on HN, it got me wondering about length. Can you get penalized for having a title that's too long? If so, how long is too long?
Anything more than about 65-70 characters will get truncated by the search engines (on Bing and Google 70 characters becomes 67 + "..."). Longer titles also look spammy to users. I usually go with 3-5 keywords plus the site name.
Browsers don't display titles in windows, they are truncated by tabs. IE9, Chrome, etc. There's little point in making them much longer. Maybe twice the length that fits in a normal unsqueezed Chrome tab, so that bookmarks can be distinguished from each other, but the one long title that maxes out the bookmark menu width is horrible too.
While I don't believe there is a "set" number, the common consensus in the SEO world is between 50-60 characters I think. Personally I have never gone over 60 in my experience, I try to keep them short, to the point, and specific to the page being displayed.
W3C used to recommend the title to be less than 64 characters in length: <a href="http://www.w3.org/Provider/Style/TITLE.html" rel="nofollow">http://www.w3.org/Provider/Style/TITLE.html</a>.