I have a question, for how long I can keep a fondant decoration (e.g a rose) specially when I am adding a gum tex to the fondant?
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.