Title tags—technically called title elements—define the title of a document. Title tags are often used on search engine results pages (SERPs) to display preview snippets for a given page, and are important both for SEO and social sharing.
The title element of a web page is meant to be an accurate and concise description of a page's content. This element is critical to both user experience and search engine optimization. It creates value in three specific areas: relevancy, browsing, and in the search engine results pages.
Primary Keyword - Secondary Keyword | Brand Name
Optimal Length for Search Engines
Google typically displays the first 50-60 characters of a title tag, or as many characters as will fit into a 512-pixel display. If you keep your titles under 55 characters, you can expect at least 95% of your titles to display properly. Keep in mind that search engines may choose to display a different title than what you provide in your HTML. Titles in search results may be rewritten to match your brand, the user query, or other considerations.
Reference - Moz.com