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 place, 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 hosting in its search results:
68-Character Title Tag: Creating Title Tags for Search Engine Optimization and Web Usability