That is a custom question that can be answered in "surprises". Here are some answers. Theoretically, you should not keep a meringue for very long. Generally, longer than 4 days is considered "good" by US standards (except under the circumstances below). Under US law, meringues can last for only so long (1 month in the worst case). It all depends on the type of meringue and how they were prepared. Meringues are very easy to store in an airtight tin. Most eclairs/shapes contain no filling and thus have no fillers, so a tin is not necessary to store them.