how far in advance can i make decorations with fondant ?

As far in advance as you'd like. A few days, weeks, whatever. Keep them in a sealed container if you want them to remain soft, or keep them in a protected but not airtight container (such as: a cardboard box) if you want them to dry out.

For me, it depends on what I'm making. If it's a topper or something that will "stand" on the cake, I'd make it a few days ahead of time and then let it sit out, so it gets a little hard. If it's something that I'm putting on the cake itself (such as stripes, polka dots), I typically do that as I decorate - that way they aren't rock hard, and still pliable if needed.