Definition: Cold Process soap needs to age or "cure" before it is finally ready to use.
Like firewood, as the soap cures, most of the water used in the recipe evaporates out of it. Cured soap is harder, milder, and more "finished."
How long to cure it?
There are really two purposes to cure:
- To make sure that the saponification process is completely complete. This generally takes about 24-48 hours. Yes, your soap really IS safe to use after a couple of days. Now, it does become milder as it ages and cures more but only a TINY bit. 99% of the ph changes happen in the first 48 hours.
- The real reason to cure your soap is for the water to evaporate and the soap to harden. A harder bar of soap will last longer, lather better and just be overall better soap. For this, yes, wait 3-6 weeks. Castile soaps or soaps with high amounts of olive oil benefit from an even longer cure. Many folks let castile soap cure for 6-8 months. After you make soap for a while, you'll be able to compare a bar that has cured for several months with one that has just cured a couple of weeks. You will be able to tell the difference!
Now, if you discount your water (use less water than your recipe calls for) you can reduce your cure time. The less water you put IN, the less time you have to wait for the water to evaporate out.
It really is only safe if you are an EXPERIENCED soap maker, though. It is NOT for beginners. Here's a good article about it. Making Soap with a Water Discount