The salt soap will start to harden almost immediately! If you're using a log mold like I used in the previous picture, you'll want to cut the soap as very soon as possible - as soon as it's firm enough to cut. The soap will still be warm even as it's going through the saponification process.
If you wait too long, the soap will be super hard, and really hard to cut. I waited just a bit too long on the bars pictured here. You can see the crumbly edges, and a couple of bars that fell apart. They're perfectly useable bars...just crumbly.