Fondant that has been opened is very hard to store. It dries so quickly. Marshmallow fondant is not any better in this area. 
You can try by wrapping it several times in saran wrap and then in an air tight ziplock bag. But don't count on it being flexible even after all of that precaution.
Now you know you can make the cake several days in advance and it will taste the same as if you made it that day? You could easily make the cake on Wednesday or Thursday and serve it on Saturday. No one but you will know. If I were wanting fondant pieces that were flexible to the cake, this is what I would do.
Now, if you talking about molding fondant shapes that do not need to bend with the cake, you can make those now and let them dry in an ant free environment. Do not cover in an airtight container. Just place them in a box and keep the light away from it.
Good luck! I'm sure someone else will have different/better advice.
Opened but unused Rolled fondant or Marshmallow fondant should be kept in an air-tight container for a maximum of 4-8 weeks. Storage time depends on many factors including commercial/homemade, weather/environment and storage conditions.
Applied rolled fondant is supposed to dry hard. The exterior surface is supposed to have the consistency of that candy-coating of a M&M - hard and "porcelain-like". The underneath remains soft and dough-like.
Don't wrap a decorated cake in plastic - condensation etc can cause the icing to melt. Store it in a cardboard box [with a cover] and place in a cool dry place. This all pre-supposes that no cake element requires refrigeration. Fondant correctly applied will totally seal that cake, keeping it fresh for up to 7 days - months if using a fruitcake.
1. Cover the cake with rolled fondant. Let this dry - 4 hours/overnight. in a covered box with a lid.
2. Model/make rolled fondant elements, eg lace, for the cake and glue/fix into position. Pipe with royal icing also at this stage. Gumpaste flowers [made in advanced] can also be arranged at this stage.
Rolled fondant is only flexible when first applied - once in the air, it dries.
I just wanted to double up on Amy's response. last weekend I made those fall cookies covered with fondant. I had a lot of brown left over. I wrapped it twice in airtight baggies then placed it in a Tupperware container and hoped for the best. I used some last night (a full 7 days later) and it was just as nice as when I placed it in the containers.
Fondant that has been opened is very hard to store. It dries so quickly.
You can try by wrapping it several times in saran wrap and then in an air tight ziplock bag. But don't count on it being flexible even after all of that precaution.
Actually, mine keeps for weeks, even months. I keep it in the original package, then saran wrap it shut. I keep it in my closet since it is the place that is the coolest and the darkest. It is just a fresh as when I bought it.
I buy a commercial fondant and I just rub a little shortening on any exposed pieces before reclosing the package as tightly as possible... it comes in a vac-sealed bag inside a box, so I just close it up tight and it lasts for weeks/months! If I've pulled off a smaller amount and coloured it, I lightly rub it with shortening (or a quick spray of Pam or similar works too) then wrap it in 2 layers plastic wrap then stick it in a freezer bag ... I often keep a bunch of little pieces this way (in the same bag)
If you have one of the food savers you can use it for fondant as well. It sucks all the air out of the packaage (which is what makes the fondant get hard). 
I wrap mine, ziplock bag, and tupperware. Then when I am ready to use I microwave it for a couple seconds at a time until nice and workable. Just don't microwave it too long. I once pushed one minure on accident and went on to do something else. HUGE MESS! 
oh..... and I use MMF too.
does anyone know why my rolled fondant WILL NOT harden? it has been two days....I live in florida and there is so much moisture and humidity...is this why? and if so i really need some help...this is my second or third attempt at making an elmo for my son's FIRST birthday cake and i am very discouraged...please advise as i only have three days before the party! Thanks in advance for responses. 
For figures and such, you are best off adding a drying or hardening agent like Gum-tex powder or tylose or something like that. I don't find for larger items they will ever harden up enough without an additive and also, you need to support larger items by inserting toothpicks or dowels if they need to stand.
You are really better off using gumpaste for larger figures or at least a mixture of 1/3 to 1/2 gumpaste mixed with fondant. I tend to use the Wilton fondant for such things and still add Gum-tex powder at the rate of about 3 tsp. per 10 ounces of prepared fondant.
A lot of people feel that in areas of high humidity such as Florida, tylose is a better hardening agent.
Remember that as you add colour to fondant you must add more icing sugar to make up for it. Also, many items will not dry in less than a week or two and still will not be hard so if you plan to stand the character up on the cake, it won't work well unless made of gumpaste or with the addition of the hardening agent.
Hugs Squirrelly
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