How soon should marshmallow fondant be used after being made?

AAfter 2 weeks at room temp I wouldnt know..but I know it can last up until then.

It should NOT be used any sooner than 8 hours after you make it

http://cakecentral.com/t/764390/marshmallow-fondant-how-long-can-it-last-before-it-is-used

use the search button in the upper right corner and enjoy reading tons of info on MMF.

It taste so good and is so simple & cheap to make, especially for kids stuff, cupcakes,  or cookies.

It depends on the recipe you use. There are some recipes that call for the fondant to rest overnight or no less than 8 hours. And there are others that are ready immediately. I've tried several recipes and there was one I found here that was ready to use right away and it was fabulous. I think it used butter instead of shortening. I will try to find it, but you should go with what works for you.