What sort of material are you using for the mold? Latex? RTV? Assuming the latter, they can last for years - I have some that I made over 10 years ago that are still supple. I let them cure for a couple of days before use - that extends the life right away, and I put them away, out of the light when I am done with them. They will of course degrade with use, depending on the complexity of the part in question - the more sharp, small edges, deep relief cuts, etc. the shorter the life.
The raw materials you make the molds out of have a definite shelf life, so be sure to use them up before they become too thick to be of use.
Good luck with your casting!
Kurt