Handmade soap is gentle and free of excessive preservatives. That fact, however, is one of the few caveats involved with making soap at home. Hand-made soap has a shelf life — a time after which it isn't as fresh as it might be.
Signs that soap has passed its shelf life vary from variety to variety. With lye soaps, an “off” or rancid odor is a sign that your soap has gone bad. If you use a high degree of lye discounting and superfatting, this will happen sooner than if you have a more completely saponified bar.
All soaps, including casting soap, will display signs of age by yellowing and changing texture from firm and smooth to warped, grainy, or slimy. Liquid and cream soaps can grow mold, particularly if not preserved. You can prevent premature spoilage through appropriate storage and careful use of cosmetic grade preservative. Every soap, though, will pass a point of no return if not used in time.