There does not appear to be any hard and fast rule when it comes to length but wordy headlines can muddy your message. It seems to be best to strike a happy medium. The following graph from http://www.outbrain.com shows the results of an analysis of 8 months worth of click through data on over 100,000 headlines.

It was found that moderate length performs best. Both shorter and longer headlines did not perform as well. Headlines with 60-100 characters earned the highest click-through rates. When measuring headline length by the number of words, rather than characters, they found the same pattern of results.

if you want your headline to show up on SERP (search engine results page) as is and don’t want it to be chopped, the suggestion is to keep it less than 70-characters.