How long will a decorated sponge cake stay fresh?

[mcfc]

I am planning on making a princess castle cake for my daughters birthday, but need to know once the sponge cake (which will be filled with jam and buttercream) stay fresh if i cover it in fondant icing as soon as it is cooled?....Basically I need to know how long I have got to decorate the cake before her birthday, as this is my first attempt I need as much time as possible, but still want the cake to be fresh? Thank you any help would be greatly appreciated