I had some that were over 10 years out of date. Of those only 1 in 3 went off.
Personally, out of date flares are better than no flares. My father only had on board other peoples expired flares. He considered them a huge waste of money with better things to spend the money on. I would agree with him.
Here is a link to the USA Federal Requirements brochure.
In it they state you may carry 'out of date flares'. P17. But 'out of date' flares cannot be included in the count of flares on board.