Most of what shortens a bar’s life happens between washes, not during them. A bar left sitting in standing water dissolves slowly from the bottom up, going soft and slick long before it is used up. The single most effective thing you can do is let it dry. A bar on a draining dish, kept clear of the shower spray, lasts two to three times longer than the same bar left in a puddle.
The short answer
A well-cured 100g bar lasts roughly four to six weeks of single-person daily use. Store it on a slatted or draining dish, lather in the hands rather than rubbing the bar on skin, and let it dry fully between washes, and the same bar can run six to eight weeks. Storage is the deciding variable.
What actually wears a bar down
Water is the agent of loss, and the amount it removes depends on a few conditions.
Storage comes first. A bar that never dries stays soft, and soft soap erodes faster. A draining dish, or even a bar stood on its edge against a wall away from the water, gives it the hours it needs to firm up again before the next wash.
Application is next. Working a lather in wet hands and then washing with that lather uses noticeably less soap than dragging the bar directly across skin. The bar is touched briefly; the hands do the rest.
Water temperature has its own effect. Hot water dissolves soap more readily than cool, so a bar that lives in the hottest part of the spray gives itself up quickly. Keeping the dish to the side, out of the direct stream, slows this.
Then there is the bar itself. A properly cured bar, one that has spent weeks losing water and hardening, is denser and more resistant than a soft, freshly made one. This is one of the quiet reasons cure time matters, and it is the same hardness that makes a good cold-process bar feel solid in the hand.
Habits that extend it
A few small adjustments compound.
Dry the bar between uses, deliberately. Shake off the surface water, set it where air reaches it on all sides, and let it sit. A draining dish does this passively; a flat, solid dish does the opposite, holding a film of water against the soap.
Rotate two bars. Use one until it needs to dry, then switch to the second while the first hardens completely. Two bars used in alternation outlast two bars used in sequence, because each gets long, uninterrupted drying time.
Trim a long bar. A tall bar worn thin in the middle snaps and frays, and the thin remnant dissolves wastefully. Cutting a fresh bar in half and using one piece at a time keeps the working surface compact and the offcut dry on the shelf.
Keep the dish away from the spray. The bar should be wet when you use it and dry the rest of the time. A dish positioned outside the direct line of the shower head makes that possible without any effort on your part.
What to expect
Frequency sets the baseline. A bar used once a day by one person follows the four-to-six-week figure; a bar shared by a household, or used for both face and body, goes faster. None of this is a flaw in the soap, it is simply how solid soap and water behave together.
The conditions that lengthen a bar’s life are also the ones that keep it pleasant to use. A bar that dries between washes holds its lather and its scent better than one left to soften. The same care that matters here matters whenever skin needs a gentler approach, washing a new tattoo, for instance, where a mild bar and a light, fragrance-free lather are the point. Once the skin has healed and is just skin again, the same storage habits apply.
Keep it dry, keep it out of the spray, and a good bar will outlast your expectations of it.