I made plaster molds for making jigs many years ago. It was a real pain in the butt and they didn't last long. The only thing that I can say good about them is that they are good for making prototypes. Other than that, try to get some aluminum molds. They are much better.
On another note, I have been using Durham's water putty to make molds for soft plastics. When baked, it makes a real hard mold. Don't know how long it would last, but I know it would be better than plaster.
Hope this info helps.
Cal