Steamers should be eaten as quickly as possible, especially during warmer months. Very fresh steamers will last a couple of days in the refrigerator, but it is usually best to eat them within 4 days after they have been cooked.
Most people will advise that you actually purchase your steamers the same day that you plan to eat them, that way you will not have to spend hours contemplating whether they have passed their proverbial sell-by-date or not.
Steamers are usually best purchased and consumed in the cold months, since they are extremely vulnerable to bacteria and so they perish really quickly.
While you can freeze steamers if you know how it is done, you should never do this after they have been cooked, since they will become very rubbery.