Making a bar last twice as long
One change does almost all of it: let it drain.
Handmade soap holds more water than a supermarket bar, which is part of why it feels different. It also means a bar left sitting in a puddle will turn to paste.
Use a draining dish. Slats, ridges, anything that lets air underneath. This single change is most of the difference between four weeks and eight.
Keep it out of the direct spray. A bar under running water dissolves whether you are using it or not.
Cut it in half. Use one piece, keep the other dry in a cupboard. The stored half also keeps hardening, so it lasts even better when its turn comes.