most will last a year if kept in airtight container in cool, dark place.
(tho -- ditch the witon and use SatinIce, Fondex or make your own MMF!
http://www.cakecentral.com/article47-How-to-Make-and-Decorate-with-Marshmallow-Fondant-MMF.html
alternate recipe: http://www.cakecentral.com/cake_recipe-3183-1-Rhondas-Ultimate-MMF.html
I've kept mine for more than 6 months and it was still good. Takes awhile to get it back to a workable consistency (for me at least). I usually just stick it in the microwave for just a few seconds to loosen it up.
HTH
Quote by @%username% on %date%
%body%