The W3C, in their page W3C - The TITLE element in HTML, states: "The title should ideally be less than 64 characters in length. That is, many applications will display document titles in window titles, menus, etc where there is only limited room." A 64 character limit is a good starting point, but oversimplifies things a bit.
Google will display up to 66 characters of a title tag, cropping to complete words. For example, the following title is exactly 68 characters, so Google leaves off the last word in the title when displaying the website in its search results:
68-Character Title Tag: Creating Title Tags for Search Engine Optimization and Web Usability Google Display: Creating Title Tags for Search Engine Optimization and Web ...
On the other hand, if the word "and" were replaced with &, the title would be exactly 66 characters, and would be displayed in full:
66-Character Title Tag: Creating Title Tags for Search Engine Optimization & Web Usability Google Display: Creating Title Tags for Search Engine Optimization & Web Usability
Google isn't the only thing to consider when deciding on the optimal title tag limit. Yahoo! has an absolute cutoff in presenting titles of exactly 120 characters, which is substantially longer and gives the webmaster much more room to present longer titles. If your title exceeds 120 characters, the title will simply be cropped, whether the display title ends in a complete word or not. That is quite different from Google, which ends the display title with the last complete word which ended before the 66 character limit was reached.
The best strategy for creating a long title is to make sure that the title works for both Google and Yahoo. The optimal title tag should be thought of as consisting of a primary title (for Google) and a secondary title (for Yahoo!). Your primary title length should be limited to 66 characters, including spaces and punctuation. Your secondary title can be any length up to the point where the full title reaches 120 characters in length. You can create a longer title if you wish, but be aware that anything beyond 120 characters will be cropped in the Yahoo! search results, and no one will ever see it.