I make it and after a week it goes rock solid. How can I have the fondant stay soft for a longer time?

Air tight storage is your only hope. Wrap in plastic, then again, then in foil....or...better yet.... get a seal-a-meal food sealer system and put it in that bag and it stays FRESH!

It might be your recipe depending on what ingredients yours calls for.

I had no luck at all with MM fondant - I hate it actually lol - but I found that the recipes that called for glucose and glycerine were much longer lasting/ more flexible etc than the ones that call just for MM's and icing sugar. So if you are storing it correctly maybe post your recipe.

I do my own MMF and it does get hard.  I just pop it in the microwave a few seconds at a time until it gets soft enough to knead but not long enough that will melt.

LOVE mine too MBalaska!

AThank you everyone. It was really helpful. Thank you for taking out the time to answer my question.

As everyone has said air tight storage is the only way. When I make MMF I lightly grease it with crisco before wrapping it in plastic wrap and then putting in a ziploc bag and then usually into a plastic container. I've never had it dry out then. I can use it weeks later & it's still fine just needs to be kneaded a little :-)