My niece is having a birthday party this weekend and I want to make a pillow cake with a cat on top. I want to make the cat out of rice crispy treats and cover with fondant. How far ahead can I do that? I would like to focus on the pillow cake, which I've never made before, this weekend and already have the cat done by then so I can just stick it on top.

Go ahead and make it now! That way you're finished w/ it, and can work on the cake part. Fondant can sit out forever, once decorated - unless you think someone will eat it, (wouldn't recommend it!)!