They will eventually become brittle and start to break apart. I have 2 sheds hanging on my cube wall. One is 3' and the other is about 4'. Both are from my first snake, and I have them here to prove better than pictures that I have snakes. They have been there for at least 4 years now and when touched you can tell they are starting to get tot he point that touching them will cause them to break. If you place them in a box that you can keep out humidity and such, they should last longer. If you want to keep them forever, you can build an air tight glass box and make it a vaccuum before totally sealing it.
I think that just about everyone that keeps snakes has at least one shed that they have kept from their snakes.