Hi Im new to cake decorating, i have made a few wedding cakes in fruit cake, and am plain mud cake flavours with the fondant covering. I am wanting to venture a little bit and try doing a sponge and mud cake with a buttercream filling . My question is. How far in advance can i fondant cover a sponge cake with a buttercream filling for a wedding and would it be okay if its not refrigerated as im worried about the fondant getting wet on the top, What do people normally do. Thanks for your advise

Fondant should not get wet unless the cake is stored COLD, then removed to warm room temp - thus developing sweat.
In general, cakes can be made & covered in fondant up to 3 days ahead. Just remember, the longer you have it, the less time the customer will have for any leftover cake to be fresh/used.