Tallow soap starts with an advantage. It's a hard bar.
Tallow is high in stearic and palmitic acid, the saturated fats that set up dense and firm. A cold-process tallow bar, fully cured, is one of the longer-lasting bars you can buy. It isn't a soft, fast-melting bar by nature.
What kills any bar early is the same thing every time. Water it isn't using.
The one rule that matters most
Let it dry between washes.
A bar left sitting in a puddle, in a slick on the shower floor, in the path of the spray, stays wet. Wet soap dissolves. It goes soft, then mushy, then gone, and the soap you paid for is down the drain doing nothing.
Lift it out of the water and let it dry, and the same bar lasts dramatically longer. That single habit is most of the battle.
How to actually do that
Use a draining dish. A soap dish with slats, ridges, or holes, so water runs off and air gets underneath. Not a flat dish that holds a pool.
Keep it out of the stream. Set it where the shower spray doesn't hit it directly between uses. A corner, a shelf, a caddy off to the side.
Give it air. Soap dries faster with airflow. A spot that breathes beats a closed, wet soap tin every time.
Two bars, alternated, beats one. If a bar gets a full day to dry out between uses, it hardens back up and lasts longer. Rotating two bars is an easy way to get that.
Buy by the case without worry
If you keep bars somewhere dry, they harden further with age, which is fine and even helpful. So having more than one on the go is no risk.
That's part of why we sell by the case. Soap doesn't spoil sitting on a shelf in a cupboard. It keeps. You're not racing a clock.
How long it keeps in storage
Use a tallow bar within about two years and you're well inside the window.
That isn't a hard expiry. Real soap doesn't go off like food. Over a long time a natural bar can slowly shift in colour or soften in character, normal for something with no synthetic preservatives, but a fragrance-free tallow bar kept dry will be fine for a long while. Buy a case, store it dry, use it as you go.
One hard bar, kept out of standing water, lasts. That's the whole trick.