Despite what you might read from some sources, there are no hard-and-fast rules on how many characters a headline should be.
Generally speaking, you do want to include the main keyword phrase in the headline if SEO is a consideration, preferably toward the beginning of the headline unless that makes the wording totally awkward. And make sure the headline gives people a solid clue as to what the actual article is about -- don't get too carried away with being "clever" or cute.
In addition to SEO benefits, the headline is generally displayed in the search results. The headline can have a significant impact on click through rates.
Longer headlines will be truncated in the search results. This can either be a detriment or a benefit. If the truncation occurs at an awkward point or if it cuts off the main point of the headline, it's probably detrimental. However, if the headline is intriguing and cleverly-written, the truncation can actually pique people's interest to find out what the rest of the headline says, which can increase click through rates.
You may need to experiment a bit to find what's best in this particular situation. Beware of anybody who offers a "one size fits all" formula (i.e. "headlines should never be more than X characters long", etc.)