I've often made them and saved them in a plastic tupperware, and they've kept for days. I think they're a pretty hearty cookie.
They need to be cooked on a day with low humidity and stored sealed . They keep for months in the freezer.
Tightly sealed/low humidity is the key. They will keep for a very long time, but will get soft then even mushy unless kept tightly sealed.
You may wish to put a tablespoon or two of cornstarch on the bottom of the dish you plan to store them in, place a paper towel on top of it to cover the base and proceed with layering the meringues. Cornstarch is highly absorbent and any potential mousture will gravitate towards it first.
Cynthia is a trusted source on Bread/Baking.
As has been suggested, low humidity is the key. I live in Montana, in an area with almost no humidity. I could practically leave meringues uncovered at room temperature. When I was in culinary school in San Francisco, I had to double-wrap them in secure plastic wrap and hope they would last 24 hours at that. The classic formula for meringue is 1 part egg whites to 2 parts sugar. Given that, the dissolved sugar is very hygroscopic, which means that in solution (dissolved), it will pull moisture out of the atmosphere. That is a long way of saying that first it depends on where you live. Second, seal them tighter then tight if you live in a humid environment. And even at that, don't plan on holding them longer than 4 or 5 days.
Cook's Illustrated just published a test disproving the idea that ambient humidity is a factor in the shelf life of meringues. Let them cool then store in an airtight container -- good for at least two weeks.
Actually, I've found that the inverse is true: the drier the ambient humidity, the harder the meringues if not adequately wrapped.
Different results could easily result from different packaging methods. From the photo that accompanied the article, CI apparently used OXO "pop" containers for their tests. Many plastic wraps are PVC-based which clings well but is significantly permeable to both air and moisture. LDPE wraps (Glad Cling Wrap, Press 'n Seal) are moisture proof.
I store meringues in an airtight container, and separate the layers with parchment paper. They keep for days that way. Over Christmas, I made beautiful meringue nests and stored them on the porch temporarily just to clear some space. I forgot about them and inadvertently left them out overnight. Needless to say, they were molten messes in the morning. They could have been used for glueing.
Ambient is ambient. Press n Seal in my experience is significantly less than moisture-proof or anything-else-proof. As for commercially or professionally sealing anything, double or triple wraps of a good, strong plastic wrap works very well. Wrapped closely and tightly, it permits very small amounts of air exposure, less than the OXO containers. And yes, we do re-use or recycle.
The fact CI successfully stored meringues for two weeks in partially-full OXO containers clearly demonstrates that air trapped inside isn't a factor. Moisture movement through a permeable barrier is, either into or out of the product depending upon ambient conditions.
In a test of plastic wraps a few years ago, CI demonstrated the relative effectiveness of PVC vs. LDPE finding the latter blocked moisture over 10 times as well as the former. PVC failed after only 2 days; LDPE was still going strong when they ended the test three weeks later.
For what it's worth, Press 'n Seal lost points only because it provides a one-time seal. That seal is so effective however that I have been able to store a pork chop for over a year in my home freezer without freezer burn.
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