I'm entering my first cake competition at the end of April ( the North Texas Cake and Sugar Art Show). I can use dummies for the cake but one part of my design I want to show can be done edible. If I use store bought rice krispy treats and cover them in candy melts, does any one know how long until the would break down? No one is going to eat it so I don't care about tasted just about it actually going bad. I want to start as early as possible I'm being pretty ambitious with my design.

I made my Godzilla and buildings about 2 weeks ago. They are still okay. Not the freshest, but still edible.

Great. I'm not at all interested in edible just stable. I've never made a cake before where taste and texture are totally irrelevant. It's a very weird feeling.

Well, in that case... I did let my 2 foot tall structure "dry out" a bit before crumb coating and covering in fondant. I wanted to make sure it would survive the 2 hour car ride (which is today!)

It's much more stable after having dried a bit.

I made a 3d baby doll three years ago for a competition. Its still fine! I used a mix of fondant and modeling chocolate over it and it dried solid and no problems. Ofcourse I dont let it get too hot in the house though!

Will gumpaste under the RKT also remain dry and stable? The gumpaste would be dry prior to the rkt being placed over it..thnks

Yes. I have had some of my competition pieces for years that I have on display. One of them is completely made out of gumpaste (a planter) with gumpaste flowers in it. And I have had that now for 6 years. No problems! But I do have air conditioning in the summer so that also helps. My mother who has some of my flowers, does not and they mildewed. She also has problems with other items mildewing because they are in the woods and the house will get very warm in the summer.
But you should be fine for a competiton piece and if you want to preserve it, use dampit or some form of silicone and keep it out of the direct sunlight.