Fondant decorations that you want to dry hard can be made months in advance as long as you store them in an air tight container and keep them away from heat and light. If you need any type of "give" for the decorations then you need to make them no more than a couple of hours before use...otherwise they get too dry and will crack when bent.
If you want to keep your fondant decorations soft so that you can form them onto the cake, you can still make them in advance; just store them in an airtight container so that they cannot dry out. They will remain pliable until you are ready to put them on the cake. I do those kind appx 1 week ahead.
When I want them to dry out I actually do NOT store in an airtight container as that traps in the moisture and inhibits drying out. Generally though, fondant is prone to crumbling as it dries out so if I want my decorations to harden, I either use gumpaste, 50/50 fondant/gumpaste, or fondant with tylose powder or gum-tex added. You can make those several weeks in advance too! (or longer as catlharper suggests, they last practically forever)