Keeping the soap dry between uses is key to making it last. After use, rinse off any surface residue and place the bar on a draining soap dish or rack that allows air to circulate underneath. Avoid letting it sit in a puddle of water in a closed soap dish, as this will cause it to soften and dissolve prematurely. A slotted wooden or plastic soap rack is ideal.
Store it in a cool, dry place away from direct splashes from a sink or shower. If you're dissolving portions in hot water for a cleaning solution, it's efficient to cut or grate off the amount you need rather than submerging the whole bar. A dense, hard soap like this will naturally have a long shelf life if stored properly, as it's not designed with additives that might degrade over time.