I have made them up to about a week or two ahead. I either set them onto wax paper and put into a deep tupperware bowl or if I don't want to risk moving it, I just invert the bowl over the top and leave it alone.
Pink and Purple are bad about fading so if you make those up early protect them from the light.
I gotta say, I much prefer using gum paste for toppers instead of fondant. Gum paste dries much, much faster.
I just did a baby shower cake and instead of using pure gum paste I went ahead and used a 50/50 mix of fondant and gum paste for a bow and that sucker was still soft after 4 days! I don't love much about Wilton...but I do love their pre-made gum paste.
I like 50/50 fondant/gum paste also, but for my Cat in the Hat, I tried fondant and Mexican Paste and it worked really well! It dried very hard, like straight gum paste, but I was able to keep the colors I needed.
http://cakecentral.com/gallery/2100654
I do toppers out of fondant all the time. 
I usually let them air dry for a few hours or overnight, so they are firm enough to handle without breaking and and can stand up in an icing swirl but still soft when you bite into them. Then I'll put them in an airtight container until I'm ready to use them. I use little individual candy ziplocs but you could put them on wax paper and put them in a bigger ziplock too. They'll last a really long time as long as you keep them sealed and away from any light source so the colors don't fade.
Here's a fondant cupcake topper I made:
http://cakecentral.com/gallery/2100765
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