Not too long ago, I was about to toss out the crescent moon shaped bit of Ogallala Bay Rum & Orange soap that remained in the bottom of my mug. Then I thought to myself “I’ll bet there is one more shave left in that”. Now that was five or six shaves ago. It’s still going strong. The same can be said for the bottle of Thayer’s Lemon Witch Hazel that has been “almost done” for the last two months, LOL.
I'll often take the last bits of a soap and press them into a new puck. It's easier for me to load a brush off a reasonably sized puck than to try loading from the small bits. If a shave soap is less than ideal and I want to use it up, I've also used it as a face soap.