When I tracked number of shaves per pick of soap, it was my experience that the amount of soap left at the time the bottom first shows through is about 30-35%.Update, the bottom of the soap appeared, time for a picture. Usually, it's not too long after that. Daily use, exceptional rinse to get a clearer picture.
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In other words, if a hypothetical shave soap had 100 shaves' worth of soap in it, you would hit the bottom somewhere around shave 65 or so. I found that surprising; my intuition was that there would have been fewer shaves remaining.