Forever and ever? Are you keeping it before putting it on cake, or taking it off a cake and keeping it as a momento?
You can make them weeks in advance before putting them on a cake and if you keep it away from direct sunlight and dust and keep them for years after taking them off.
Well, the thing is that you're not really going to eat the flowers, and they won't go 'off' as such, so you could probably make them weeks, or even a couple of months in advance. I've had decorations in my cupboard for maybe 6 weeks before.
The only problem is that fondant + gumtex flowers get quite brittle over time, so you might pull them out to use them and find that all the petals break at the slightest touch. Gumpaste keeps better in that way, I've found. Someone might be able to tell you more.
Hey, your post seems like you are really worried !!
You can keep it Forever!! :) :)
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