Hi dlgwe,
First off, welcome to CC 
You can keep MMF for a while in an airtight bag and then placed in an airtight container out of direct sunlight. I usually keep mine for about a month. Unfortunately, you can not freeze it.
Before reusing, nuk it in the microwave to soften some for about 15 to 20 seconds to...depending on the amount that you are using.
HTHs 
I've kept some MMF for 2 months and it was just fine .. as long as it's sealed really tight and no air gets to it, if the smell and taste is good, then you can use it .. it's just melted marshmallows, water and flavoring.. nothing much to spoil...
Welcome to CC 
Glad to have you here with us!!
You can't freeze MMF? I've been freezing all of my MMF leftovers. Most of them seem okay after they thaw and I use them. But I notice that even though I grease the rolling pin and mat, it'll still stick. Not sure if that's because it was once frozen or if it was just that particular batch.
There are some great tips here....
http://www.baking911.com/asksarahbb/index.php?showtopic=1100
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