Unless you have given up boating, keep your flares - you can never have enough if/when you get into trouble. Legally you need (I think) 3 flares. My opinion is that you will go through 3 flares very quickly, especially if it's in the middle of the night when few boats are out. However, you need to store them separately from your legal flares on the boat.

I probably have 15 years of expired flares in a box labeled "expired".