want to use RKT to make some models for a cake, never used it before so can someone tell me how far in advance i can make the models and cover with fondant
You can make them as far in advance as you want, since they won't go bad for months. Once the fondant has set, just store them away from the light and dust, but not in anything airtight. A cardboard box is ideal. To give the fondant time to set, you should probably make them at least 2-3 days in advance, although now that I think about it, I've made them and stuck them directly on the cake when they were done (nothing like leaving things till the last minute)!
Marianna, I am planning on using the RKT recipe that's on the box. It's says to "store no more than two days in airtight container." The recipe is 3 tablespoons butter, 10oz marshmallows, 6 cups Rice Krispies. Do I need to change anything to make these last longer than two days? Or will this recipe stay edible longer than two days?
I'm not sure of the difference (if any), but I would search the recipes section on this site for a good, cake friendly RKT recipe. I believe I have seen one...that way you can be sure.
I have to confess that I use store-bought RKTs, and they're probably full of preservatives. I think you should do what instant-gratificaketion suggests. I generally don't make home-made RKTs because the marshmallows you get in Mexico have much more gelatin in them and so the RKTs come out a lot harder than the ready-made ones and are really hard to work with.