You really can't re-use fondant effectively after it's been on a dummy for over a few hours. Once it's exposed to air, it crusts (sugar crystalizing) and if you peel it off and knead it back together, you'll have lots of hard bits and it would likely be crumbly and hard to keep together.
I make my dummies for competitions and then take them to days of sharing for ICES or use them examples of my work when I donate cake certificates to fundraising charities. I often keep them for up to 2 years, so peeling them to re-use the fonant isn't really an issue.
As a side note, I used to use water to adhere the fondant, but another decorator suggested using the crisco. It takes a bit more time, but I find that the crisco fills in lots of tiny imperfections in the dummies. With water, I noticed that the water would sit in the imperfections, the fondant would sort of "suck into them", and when it dried, the imperfections were actually magnified.
HTH
Rae
You really can't re-use fondant effectively after it's been on a dummy for over a few hours. Once it's exposed to air, it crusts (sugar crystalizing) and if you peel it off and knead it back together, you'll have lots of hard bits and it would likely be crumbly and hard to keep together.
I make my dummies for competitions and then take them to days of sharing for ICES or use them examples of my work when I donate cake certificates to fundraising charities. I often keep them for up to 2 years, so peeling them to re-use the fonant isn't really an issue.
As a side note, I used to use water to adhere the fondant, but another decorator suggested using the crisco. It takes a bit more time, but I find that the crisco fills in lots of tiny imperfections in the dummies. With water, I noticed that the water would sit in the imperfections, the fondant would sort of "suck into them", and when it dried, the imperfections were actually magnified.
HTH
Rae
To avoid the imperfections you can sand the dummy. But the crisco does work too.
Edna 
Oh, I do sand the dummies--I hate it during the winter when I can't go outside and do it--it's soooooo messy--and static electricity takes the bits everywhere. I made my own Lambeth dummies one time and the inside of garage looked like it had snowed all over it! 
I really prefer floral foam dummies to pebble foam dummies, but the floral foam ones are almost impossible to come by in larger sizes. I've made my own, but it's an awful lot of work.
I've noticed recently that many of the pebble foam dummies I get have hard "lines" and really "plasticky" bits of foam in them. I think it's the result of so much recycling of products. These contaminents can make really smooth sanding really hard.
It's usually with my (probably) over-used, re-used dummies that I have divots to fill--that's where the crisco works really well. If I sanded those down to where they were really smooth, I'd have styro cupcakes 
Rae
Oh, I do sand the dummies--I hate it during the winter when I can't go outside and do it--it's soooooo messy--and static electricity takes the bits everywhere. I made my own Lambeth dummies one time and the inside of garage looked like it had snowed all over it!
I really prefer floral foam dummies to pebble foam dummies, but the floral foam ones are almost impossible to come by in larger sizes. I've made my own, but it's an awful lot of work.
I've noticed recently that many of the pebble foam dummies I get have hard "lines" and really "plasticky" bits of foam in them. I think it's the result of so much recycling of products. These contaminents can make really smooth sanding really hard.
It's usually with my (probably) over-used, re-used dummies that I have divots to fill--that's where the crisco works really well. If I sanded those down to where they were really smooth, I'd have styro cupcakes
Rae
lol..yes reused dummies will show the age and same goes for the floral dummies. The styrofoam has more open pores that they do show if you dont cover..But at the end of the day..all can be fix!..
Edna 
I really prefer floral foam dummies to pebble foam dummies, but the floral foam ones are almost impossible to come by in larger sizes. I've made my own, but it's an awful lot of work.
Rae
How exactly does that work out for you? I thought of using the floral foam but really wasn't sure if it was a good idea. Do you rub those down in crisco also?
For smaller cakes, I think it works very well. For large ones (over about 12" across), too much piecing.
Yes, I wipe those down with crisco, too.
Rae
I have a display in my cafe that I did over two years ago......it's holding up strong. Cake dummies shouldn't be a lot of work to do. Cover the styro in fondant on move on from it. Here's a fun one for you.......set them outside in a rainstorm to when you want to wash the fondant off...or under the sprinklers! It's a display, nobody is going to eat it!! It's just a tad dry
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